BEIRUT -- Syrian rebels brought their fight within a mile of the heart of Damascus on Friday, seizing army checkpoints and cutting a key highway with a row of burning tires as they pressed their campaign for the heavily guarded capital, considered the likely endgame in the nearly 2-year-old civil war.
The clashes raised fears that Damascus, a major cultural center and one of the world's oldest continuously inhabited cities, could fall victim to a protracted battle that would bring the destruction seen in other major cities and trigger a mass refugee exodus into neighboring countries.
"Any attempt by the rebels to advance into central Damascus would mean the beginning of a very long fight," said Syrian activist Rami Jarrah. "I imagine Aleppo would be a small example of what is likely to happen in Damascus."
Aleppo, Syria's largest urban center and main commercial hub, has been convulsed by violence since the summer, when rebels launched an offensive to take control of the city. Since then the fighting has been locked in a deadly stalemate, with the war-ravaged city carved up into government- and opposition-held strongholds.
The latest Damascus offensive, launched from the northeastern side of the city, did not appear to be coordinated with rebels on other sides of the capital, and it was unclear whether the opposition fighters would be able to hold their ground.
Previous attempts to advance on the capital have failed. The government controls movement in and out with a network of checkpoints, and rebels have failed so far to make significant inroads.
In Geneva, the U.N. refugee agency reported a major increase in the number of people fleeing Syria, with 5,000 refugees crossing the borders daily into neighboring countries. The mass exodus "is really a full-on crisis," agency spokesman Adrian Edwards said.
Some 787,000 Syrians are registered as refugees, mainly in Lebanon, Iraq, Jordan and Turkey, Edwards said -- a number that has shot up 25 percent in January alone.
A rebel advance on Damascus, which has largely been spared the destruction of other cities, is likely to trigger a fresh wave of refugees into Jordan and Lebanon, where resources are already stretched to the breaking point.
Syria's crisis began in March 2011 with largely peaceful protests inspired by the Arab Spring revolts elsewhere in the region that toppled longtime Arab dictators. It evolved into a civil war as the opposition took up arms to fight a government crackdown on dissent.
The latest fighting in Damascus, some of the heaviest to hit the city since July, began Wednesday with a series of rebel attacks on regime checkpoints along a key road from Damascus to northern Syria. Opposition fighters and government forces have been clashing in the area since.
On Friday, rebels shut down the highway out of the capital for several hours, activists said.
Online videos showed a row of burning tires blocking all traffic as fighters with automatic rifles patrolled the area. Smoke rose up from a number of areas nearby, reflecting clashes and government shelling. The videos appeared genuine and corresponded to activist reports.
A spokesman for one of the opposition groups fighting in the area said the rebels sought to open a path for a future assault on the city.
"This is not the battle for Damascus. This battle is to prepare for the entry into Damascus," he said via Skype, giving only his nickname, Abu al-Fida, for fear of reprisals.
The city is heavily fortified and activists say it is surrounded with three of the most loyal divisions of the army, including the Republican Guard and the feared 4th Division, commanded by President Bashar Assad's brother Maher.
Friday's fighting revolved around the capital's main highway heading toward the country's north. Abu al-Fida said one checkpoint changed hands twice on Thursday but was securely in rebel hands Friday. He said rebels were within a mile of Abbasid Square in central Damascus and were firing mortars at a military base near the landmark plaza.
The Britain-based Syrian Observatory for Human Rights reported clashes in Jobar and shelling and airstrikes on the nearby areas of Zamalka and Qaboun. Rebels also battled government troops in the southern neighborhood of Yarmouk, as well as in the rebel-held suburbs of Daraya and Moadamiyeh, where six people died in a government shell attack, it said.
Meanwhile, dramatic footage of the shelling of a village in central Homs province on Thursday showed people running and screaming in panic, carrying away children and injured as explosions reverberated and smoke rose from buildings. Areas in Homs were still being targeted on Friday.
Also Friday, the Observatory said 54 people were killed, including 11 women, in a bombing at a bus stop near a military factory earlier in the week.
Observatory director Rami Abdul-Rahman said an explosive-laden mini-bus blew up at a bus stop outside the factory in Buraq, near the central city of Hama, while workers were waiting for rides home. The factory makes military supplies, but not weapons, he said.
The area is government-controlled, which is why reports on the blast were slow to emerge.
"These people work for the Ministry of Defense, but they are all civilians," he said, adding that no one from the military was killed in the blast.
Syria's state news agency said "terrorists" detonated a car bomb near the factory. The regime refers to rebels fighting to topple the Assad regime as terrorists.
No one has claimed responsibility for the blast, which resembled others in recent months that appeared to target buildings associated with Syria's military and security services.
Some of the bombings have been claimed by an al-Qaida-linked group fighting alongside the rebels, Jabhat al-Nusra, which the U.S. had designated a terrorist organization.
As the situation in Syria has worsened, foreign jihadists have flocked to Syria to join what they consider a holy war to replace Assad's regime with an Islamic state.
Late Thursday, the chief of the Netherlands' top intelligence agency warned that dozens of Dutch citizens are fighting with Syria's rebels and could return home battle-hardened and radicalized.
General Intelligence and Security Service chief Rob Bertholee told the Dutch show Nieuwsuur that hundreds of people from around Europe and dozens from the Netherlands have travelled to Syria to join rebels fighting Assad. He said propaganda romanticizing the civil war is helping draw foreigners into Syria's maelstrom of violence.
Meanwhile, in the northern town of Saraqeb, a fistfight broke out between Islamist rebels and more moderate protesters at an anti-Assad rally Friday, highlighting a growing divide between opposition forces fighting in Syria.
A video posted online by activists showed protesters marching, some carrying black banners favored by the Islamists and others carrying the black, green and white rebel flag.
The fight broke out after some protesters tried to take down the rebel flag. A shouting match ensued, with some shouting, "The people want a civil state!" and others trying to drown them out with chants of "The people want an Islamic Caliphate!"