BEIRUT - Syrian rebels including Islamic extremists took full control of a sprawling military base Tuesday after a bloody two-day battle that killed 35 soldiers, activists said. It was the latest gain by opposition forces bolstered by an al-Qaida-linked group that has provided skilled fighters but raised concerns in the West.

The Sheik Suleiman military base was the second major base captured in the north by the rebels, who also are making inroads farther south toward the capital, Damascus.

In other violence, dozens of people were reported injured or killed in Aqrab, a village in central Hama province, in a series of explosions. The Britain-based Syrian Observatory for Human Rights reported the bloodshed, citing activists in the area, but had no immediate death toll or details on who was to blame.

Fighters from jihadi groups, including Jabhat al-Nusra, were among those doing battle in the rebel ranks as they took control of Sheik Suleiman base, near the northern city of Aleppo, according to the Observatory and other activists.

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.

But al-Nusra fighters appear to be among the most effective fighting forces on the rebel side, spearheading many of the recent gains.

The U.S. terror designation freezes any assets members of al-Nusra may have in U.S. jurisdictions and bars Americans from providing the group with material support. It's 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.

The administration took further action Tuesday against extremists on both sides, with the Treasury Department setting separate sanctions against two senior al-Nusra leaders and two militant groups operating under the control of Syrian President Bashar Assad's government. Two commanders of the pro-regime shabiha force also were targeted.

"We will target the pro-Assad militias just as we will the terrorists who falsely cloak themselves in the flag of the legitimate opposition," said David S. Cohen, the department's sanctions chief.

The battle for Sheik Suleiman military base ended when the rebels took over the site's main compound and warehouses that housed a military research center. They had first breached the base perimeter on Sunday afternoon, after weeks of fighting with soldiers loyal to Assad, according to the Observatory, which relies on a network of activists inside Syria. The Observatory said 35 soldiers were killed but did not give figures on rebel casualties.

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 reports of violence could not be confirmed as the government restricts independent reporting in the country.

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. Last week U.S. officials said there was evidence that Syrian forces had begun preparing sarin, a nerve agent, for possible use in bombs.

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.

Speaking to reporters flying with him from Washington to Kuwait, Panetta suggested the threat was no longer escalating, although he was not specific about any Syrian military preparations.

"At this point the intelligence has really kind of leveled off," he said. "We haven't seen anything new indicating any aggressive steps to move forward in that way."

Asked whether he believed Assad was heeding Western warnings against using chemical weapons, Panetta said: "I like to believe he's got the message. We've made it pretty clear. Others have as well."

He noted that the Assad regime is coming under increasing pressure from rebel forces.

"Our concern is that if they feel like the regime is threatened with collapse, they might resort to these kinds of weapons," he said.

Syria is believed to have a formidable arsenal of chemical weapons, including sarin and mustard gas, although its exact dimensions are not known. Syria is not a signatory to the 1997 Convention on Chemical Weapons and thus is not obliged to permit international inspection.

The government in Damascus has been careful not to confirm it has chemical weapons, while insisting it would never use such weapons against its own people.

"Syria doesn't own any internationally banned weapons, whether chemical, nuclear or biological," Syrian Information Minister Omran 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 while the administration tries to blunt the influence of extremists.

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.

Also Tuesday, the U.N. refugee agency said the number of Syrian refugees registered by the United Nations in the Middle East and North Africa has surpassed half a million.

The figure is climbing by more than 3,000 per day, UNHCR said. According to UNHCR's latest figures from Lebanon, Jordan, Iraq, Turkey and North Africa, more than 500,000 Syrians are either already registered or in the process of being registered.

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Associated Press National Security Writer Robert Burns contributed to this report from Kuwait City. AP writer Barbara Surk contributed to this report from Sidon, Lebanon.