Heavy clashes between rebels, troops at military air bases in northern Syria

Syrian rebels seeking to topple President Bashar Assad fought intense battles with his troops on Sunday at two military air bases in northern Syria, activists said.

The fighting raged inside the sprawling Abu Zuhour air base in northwestern Idlib province and the Kweiras military air base in northern Aleppo province.

The Britain-based Syrian Observatory for Human Rights said at least seven fighters were killed in the fighting in Abu Zuhour, in addition to an unknown number of soldiers. The group, which relies on a network of activists on the ground, said the Syrian air force conducted an airstrike on Abu Zuhour village during the fighting to ease pressure on government troops inside the air base.

Rebels control much of Idlib and Aleppo provinces, which border Turkey, although government troops still hold some areas including the provincial capital of Idlib province and parts of the city of Aleppo, Syria's largest urban centre.

The fighters entered the two air bases on Saturday. Both have been under siege for months.

Syria's conflict started with largely peaceful anti-government protests in March 2011 but eventually turned into a civil war. More than 70,000 people have been killed, according to the United Nations.

The Obama administration said Thursday that intelligence indicates that government forces likely used chemical agents against rebels in two attacks.

Washington's declaration was its strongest on the topic so far, although the administration said it was still working to pin down definitive proof of the use of chemical weapons. It held back from saying Damascus had crossed what President Barack Obama has said would be a "red line" prompting tougher action in Syria.

Both sides of the civil war accuse each other of using the chemical weapons.

The deadliest such alleged attack was in the Khan al-Assal village in the Aleppo province in March. The Syrian government called for the United Nations to investigate alleged chemical weapons use by rebels in the attack that killed 31 people.

Syria, however, has not allowed a team of experts into the country because it wants the investigation limited to the single Khan al-Assal incident, while U.N. Secretary-General Ban Ki-moon has urged "immediate and unfettered access" for an expanded investigation.

The state-run al-Thawra newspaper on Sunday accused the U.N. secretary general of being a "tool" for the United States and accused him of "bowing to American and European pressures."

In neighoring Lebanon, Hezbollah's Al-Manar TV reported that Russia's Deputy Foreign Minister Mikhail Bogdanov met Saturday night with the pro-Syrian militant group's leader, Sheikh Hassan Nasrallah. No details emerged of the late night meeting.

The Shiite Muslim group has been drawn into the fighting in Syria and is known to be backing regime fighters in Shiite villages near the Lebanon border. The Syrian opposition accuses fighters from the group of taking part in the Syrian military crackdown inside the country.

At a Sunday morning at a news conference in Beirut, Bogdanov called for a diplomatic solution to Syria's civil war based on the Geneva Communique of June 2012. The communique is a broad but ambiguous proposal endorsed by Western powers and Russia to provide a basis for negotiations.