BEIRUT (AP) — Syrian rebels attacked a sprawling military air base in the country’s northwest on Saturday, while in the south opposition forces assaulted a string of army checkpoints and positions, activists said.

The raids follow nearly two weeks of advances by Syrian troops, mostly in the suburbs of the capital, Damascus, and areas near the Lebanese border in the central province of Homs.

In Moscow, Syria’s Information Minister Omran al-Zoubi denied recent U.S. charges that Syrian troops used chemical weapons against the rebels, saying Washington had leveled the accusation as a result of the latest victories by the army.

"The American hysteria about the use of chemical weapons was caused by the success of the Syrian Arab Army in striking terrorists," al-Zoubi was quoted by state TV as saying. He was using the term that the government uses to refer to rebels.

The Obama administration said Thursday that intelligence indicates government forces likely used the nerve gas sarin 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 weapons’ use. It held back from saying Damascus had crossed outright what President Barack Obama has said would be a "red line" prompting tougher action in Syria.

The rebels accuse regime forces of firing chemical agents on at least four occasions since December, killing 31 people in the worst of the attacks. They say world inaction would only encourage Assad to use the weapons on a larger scale.

The regime countered that it was the rebels who fired chemical weapons — pointing to their capture of a chemical factory last year as proof of their ability to do so.

In Saturday’s fighting at the Abu Zuhour air base in northwestern Idlib province, the Britain-based Syrian Observatory for Human Rights said there were casualties on both sides. The base has been under rebel siege for months.

The Observatory and the Local Coordination Committees said the Syrian air force conducted several air raids during the fighting to ease pressure on government troops inside the air base.

State-run news agency SANA quoted a military official as saying the troops repelled the attack and inflicted "great losses" on the attackers.

Rebels control much of Idlib province, which borders Turkey, although government forces still hold some areas, including the provincial capital that carries the same name.

Elsewhere, the Aleppo Media Center said rebels had entered the Kweiras military air base in Aleppo province and destroyed its operations room. The base has also been under siege for months.

In the southern province of Daraa, also known as the Houran plains, the Observatory and the LCC said rebels had launched a new offensive called "the Houran Volcano" in which they are targeting army checkpoints and positions.

The Observatory said there were losses on both sides but had no immediate breakdown.

An amateur video posted online showed rebel artillery fire hitting al-Khudr military base, located on a hill near the town of Dael, also in the province. Cracks of gunfire could be heard from a distance.

The video appeared genuine and corresponded to other AP reporting on the events depicted.

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.