BEIRUT -- Syrian rebels backed by Islamic extremist fighters took full control of a sprawling military base Tuesday after a two-day battle that killed at least 35 government troops, an activist group said.
It was the second major base captured in the country's north by the rebels, who have been racking up victories in the area in recent weeks and making inroads farther south toward Damascus, seat of the government they are fighting to overthrow.
Fighters from jihadi groups including the al-Qaida-linked Jabhat al-Nusra were among those doing battle in the rebel ranks, according to the Britain-based Syrian Observatory for Human Rights, which relies on a network of activists inside Syria. Al-Nusra fighters appear to be among the most effective fighting forces on the rebel side, spearheading many of the recent gains.
The presence of the jihadi groups has raised concerns in the U.S. and other nations that are supporting the opposition in Syria but do not want to see extremists gain power in the region. The U.S. this week blacklisted al-Nusra as a foreign terrorist organization and said the group was part of al-Qaida in Iraq.
The battle for the sprawling Sheik Suleiman military base, near the northern city of Aleppo, ended when the rebels took over the site's main compound and warehouses that housed a military research center, according to the Observatory. They had first breached the base perimeter on Sunday afternoon, after weeks of fighting with soldiers loyal to President Bashar Assad.
The Observatory said 35 soldiers were killed but did not give figures on rebel casualties from the battle.
Also Tuesday in Aleppo -- the country's largest city and commercial center -- four mortar rounds hit the predominantly Kurdish neighborhood of Sheik Maksoud, killing 11, including three children and two women, and wounding a dozen other people, the Observatory said.
The conflict started nearly 21 months ago as an uprising against Assad, whose family has ruled the country for four decades. It quickly morphed into a civil war, with rebels taking up arms to fight back against a bloody crackdown by the government. According to activists, more than 40,000 people have been killed since March 2011.
Western officials have raised concerns that an increasingly desperate Assad might unleash his chemical weapons stockpiles against rebels in an act of desperation.
But U.S. Defense Secretary Leon Panetta said Tuesday the Syrian government seems to have slowed preparations for the possible use of chemical weapons against rebel targets. Last week U.S. officials said there was evidence that Syrian forces had begun preparing sarin, a nerve agent, for possible use in bombs.
Panetta told reporters flying with him from Washington to Kuwait that the threat was no longer escalating, although he was not specific about any Syrian military preparations. He said the U.S. hasn't seen "anything new indicating any aggressive steps to move forward."
Panetta says he'd like to believe Assad "got the message" when other countries warned against using chemical weapons. But he's still concerned that if "the regime is threatened with collapse, they might resort to these kinds of weapons."
Syria has been careful not to confirm it has chemical weapons, while insisting it would never use such weapons against its own people.
Syrian Information Minister Omran al-Zoubi said in an interview aired late Monday that Damascus doesn't possess any internationally prohibited weapons.
"Syria doesn't own any internationally banned weapons, whether chemical, nuclear or biological," al-Zoubi told Al-Manar TV, a station owned by the Lebanese militant group Hezbollah, which is a Syrian ally. "Even if Syria possessed such weapons, it will not use them for moral reasons."
He said Western statements are similar to those that preceded the 2003 invasion of Iraq that accused Saddam Hussein of hiding weapons of mass destruction. After the U.S.-led invasion, no such weapons were found.
The Obama administration is getting ready to tighten its ties to Syria's main opposition group, the newly formed Syrian National Coalition for Opposition and Revolutionary Forces, at an international conference on the crisis in Morocco this week. The move will pave the way for greater U.S. support for those seeking to oust Assad.
The conference on Wednesday comes just days after the U.S. blacklisted al-Nusra, freezing any assets its members may have in U.S. jurisdictions and barring Americans from providing the group with material support.
The designation is largely symbolic because the group is not thought to have holdings or support in the United States, but officials hope the penalties will encourage others to take similar action and discourage Syrians from joining.
Jabhat al-Nusra is a shadowy group with an al-Qaida-style ideology whose fighters come from Libya, Tunisia, Egypt, Saudi Arabia, Iraq, the Balkans and elsewhere. Many are veterans of previous wars who came to Syria for what they consider a new "jihad" or "holy war" against Assad.
But several hundred fighters from Jabhat al-Nusra -- Arabic for "the Support Front" -- have also been a valued addition to rebel ranks in the grueling battle for control of Aleppo. The group also has claimed responsibility for suicide bombings on Syrian government targets.
Jabhat al-Nusra is the largest grouping of foreign jihadis in Syria, and the rebels say they number about 300 fighters in Aleppo, as well as branches in neighboring Idlib province, the city of Homs and Damascus. U.S. and Iraqi officials also have said they believe members of al-Qaida's branch in Iraq have crossed the border to join the fight against Assad.
The Observatory also reported clashes and shelling in several Damascus suburbs, including Aqraba, near the international airport where fighting started early this month.
The Local Coordination Committees, another activist group, said five people were killed during clashes between troops and rebels in the Damascus suburb of Rankous.
The Observatory and the LCC reported casualties in clashes in the northwestern Jisr al-Shughour area near the border with Turkey.