BEIRUT -- Turkish artillery fired on Syrian targets Wednesday after shelling from Syria struck a border village in Turkey, killing five civilians, sharply escalating tensions between the two neighbors and prompting NATO to convene an emergency meeting.
"Our armed forces at the border region responded to this atrocious attack with artillery fire on points in Syria that were detected with radar, in line with the rules of engagement," the Turkish government said in a statement from the prime minister's office.
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The artillery fire capped a day that began with four bombs tearing through a government-held district in Syria's commercial and cultural capital of Aleppo, killing more than 30 people and reducing buildings to rubble.
Along the volatile border, a shell fired from inside Syria landed on a home in the Turkish village of Akcakale, killing five civilians, including some children, and wounding a dozen others according to Turkish media.
The shelling appeared to come from forces loyal to Syrian President Bashar Assad's regime, which is fighting rebels backed by Turkey.
"Turkey, acting within the rules of engagement and international laws, will never leave unreciprocated such provocations by the Syrian regime against our national security," according to the terse statement from the office of Prime Minister Recep Tayyip Erdogan.
Secretary of State Hillary Rodham Clinton said the U.S. was "outraged that the Syrians have been shooting across the border," adding that she would speak with Turkish Foreign Minister Ahmet Davutoglu on the matter.
"It's a very, very dangerous situation," Clinton said. "And all responsible nations need to band together to persuade the Assad regime to have a cease-fire, quit assaulting their own people and begin the process of a political transition."
NATO's National Atlantic Council, which is composed of the national ambassadors, held an emergency meeting in Brussels on Wednesday night at Turkey's request to discuss the cross-border incident. Turkey was likely to receive an expression of support from the alliance, although any imminent move by the NATO members to intervene militarily seems remote. NATO also held an emergency meeting when a Turkish jet was shot down by Syria in June, killing two pilots.
A NATO official, who spoke on condition of anonymity because of NATO rules, said the meeting in Brussels was being held under a treaty article that states "the parties will consult whenever, in the opinion of any of them, the territorial integrity, political independence, or security of any of the parties is threatened."
Turkey, a NATO ally, wants to avoid going into Syria on its own. It has been pushing for international intervention in the form of a safe zone, which would likely entail foreign security forces on the ground and a partial no-fly zone. However, the allies fear military intervention in Syria could ignite a wider conflict, and few observers expect robust action from the United States, which Turkey views as vital to any operation in Syria, ahead of the presidential election in November.
Turkey, which has moved military reinforcements to the border in recent months, has more than 90,000 Syrian refugees in camps along its border, and also hosts Syrian opposition groups.
There is concern in Turkey that the Syrian chaos could have a destabilizing effect on Turkey's own communities; some observers have attributed a sharp rise in violence by Kurdish rebels in Turkey to militant efforts to take advantage of the regional uncertainty.
"This latest incident has caused the glass to spill over too much," said Turkish Deputy Prime Minister Bulent Arinc.
"Turkey is a sovereign country. There was an attack on its territory. There must certainly be a response in international law. ... I hope this is Syria's last craziness. Syria will be called into account," he said.
There was no immediate claim of responsibility for the Aleppo bombings, but the government blamed its opponents, saying the huge explosions were caused by suicide attackers. The technique is a signature of al-Qaida-style jihadist groups, some of which are known to have entered Syria's civil war to fight against the regime.
"It was like a series of earthquakes," a shaken resident told The Associated Press, asking that his name not be used out of fear for his personal safety. "It was terrifying, terrifying."
The Syrian government said the bombings killed 34 people and injured 122 -- although death tolls have been difficult to verify. The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground, said at least 40 people were killed.
The state-run Ikhbariya TV channel showed massive damage around Saadallah al-Jabri Square, which also houses a famous hotel and a coffee shop that had been popular with regime forces. One building appeared to have been leveled and the facade of another was torn away.
The station broadcast video of several bodies, including one being pulled from a collapsed building. Rescuers stood atop piles of concrete and debris, frantically trying to pull out survivors.
Activists could not reach the area, which is controlled by security forces and sealed off with checkpoints.
The uprising against Assad began in March 2011 and gradually became a bloody civil war. The conflict has killed more than 30,000 people, activists say, and has devastated entire neighborhoods in Syria's main cities, including Aleppo.
Syria's government has always blamed the uprising on what it calls foreign terrorists, despite months of peaceful protests that turned violent after repeated attacks by security forces. The transformation of the conflict into an open war has given an opportunity to foreign fighters and extremists, analysts say.
The Syrian opposition denies any links to terrorists, but a Sunni extremist group called Jabhat al-Nusra, or Victory Front, has claimed responsibility for bombings in the past.
After Wednesday's blasts, regime forces unleashed shelling on rebel-held areas and fired machine guns from aircraft, according to an Associated Press journalist in Aleppo, Syria's largest city with a population of 3 million.
At least 15 people wounded by shelling arrived with serious injuries at the city's Shifa Hospital. All but one were civilians. Three bodies -- an old man, a woman and a middle school-age boy -- also were taken to the hospital.
Rebel fighters, many with only light weapons, advanced slowly, moving building by building. The heavier weapons, such as rocket-propelled-grenade launchers, were sent to the front lines to prevent the regime from retaking areas seized by rebels in the past two months.
Wednesday's attacks were the latest turn in the deadly -- and increasingly chaotic -- fight for control of Aleppo, one of the world's oldest continuously inhabited cities.
Long free of the violence that has engulfed much of the rest of the country in the first year of the uprising, Aleppo was struck by two suicide car bombings at security compounds in February, killing 28 people. Such attacks targeting security agencies and soldiers have become common in Syria, particularly in the capital, Damascus.
In the past two months, Aleppo has become a key battleground. The opposition launched an offensive on the city in July, and large swaths have been shattered.
Rebels last week announced a new push to capture Aleppo, which would be a major strategic prize and give the victor new momentum. It also would provide the opposition with a base and easy logistical supply lines with Turkey to the north that would allow them to carry out their fight against the regime in the rest of the country.
Over the weekend, a fire sparked by fighting spread through Aleppo's centuries-old covered market in the Old City, burning more than 500 shops. At 12 kilometers (7.5 miles), it is the Middle East's longest souk and was added to UNESCO's list of World Heritage sites in 1986.
Aleppo-based activist Mohammad Saeed said Wednesday's blasts went off minutes apart and appeared to be car bombs and were followed by clashes and heavy gunfire.
"The area is heavily fortified by security and the presence of shabiha," he said, referring to pro-Assad gunmen. "It makes you wonder how car bombs could reach there."
Syrian state TV said three suicide bombers detonated cars packed with explosives in Saadallah al-Jabri Square, near an officers' club. The square holds symbolic importance for residents because it is named after a Syrian independence fighter who resisted French occupation.
Activists and Syrian state media said a fourth car bomb went off a few hundred meters (yards) away in the Bab Jnein area near the Old City. It was not immediately clear how many casualties there were from that blast.
The Syrian Observatory for Human Rights said mortars also targeted the nearby political security department around the same time of the bombings.
Syria's Interior Ministry vowed to "track down the perpetrators anywhere." The speaker of the Syrian parliament, Mohammad Jihad al-Lahham, told the assembly that he condemns "the countries that conspire against Syria and stand behind the terrorists."
Israel also is concerned that fighting from Syria's civil war could spill across the border. Last month, mortar shells exploded in Israeli-controlled territory. Nobody was hurt and Israel said the shells were misfired.
On Wednesday, the Israeli military said dozens of armed men have gathered on the Syrian side of the frontier in the Israel-controlled Golan Heights. Authorities closed a tourist site there in response.
An Israeli spokesman said it was unclear what the armed Syrians were doing. He said there was no violence, and a tour group left Mount Hermon without incident. The army was watching the area. He spoke on condition of anonymity under military rules.
Israel captured the Golan Heights from Syria in the 1967 Mideast war. It later annexed the strategic territory in a move that is not recognized internationally.
Torchia reported from Istanbul Associated Press writers Zeina Karam in Beirut; Manu Brabo in Aleppo, Syria; Albert Aji in Damascus, Syria; Don Melvin in Brussels; and Bradley Klapper in Washington contributed to this report.