Syria's Information Minister Omran al-Zoubi denied on Saturday 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. The government refers to rebels as "terrorists."

The Obama administration said Thursday that intelligence indicates that 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 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.

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 siege from the rebels 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.

The 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 of 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 been under siege for months.

The media center said battles inside the air base continued Saturday afternoon and that the Syrian air force had bombarded the facility.

The Observatory said a rebel commander, who headed the Two Shrines Brigade, was killed in the fighting around the base. It added that six government troops were also killed in the clashes.

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 an unknown number of casualties on both sides.

The Observatory reported shelling and clashes in other areas, including Damascus and its suburbs as well as the central province of Homs and Deir el-Zour to the east that borders Iraq.

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 New York-based Human Rights Watch said Saturday that recent Syrian government air and missile strikes have caused civilian casualties in opposition-held areas in Aleppo province "in violation of the laws of war." It said the attacks left at least 84 people dead.

The rights group said its team in the province, which investigated recent attacks, said dozens of civilian homes also had been destroyed "without damaging any apparent opposition military targets."

During a recent seven-day mission to Aleppo, Human Rights Watch researchers documented five attacks between March 18 and April 7. It said that of the 84 killed in these attacks, 36 were children.

"In all the new cases, witnesses told Human Rights Watch that the only people killed or injured by the strikes had been civilians, and that only civilian buildings had been hit," the group said.

"In attack after attack in Aleppo, it is only civilians and civilian homes that are hit by government airstrikes," said Anna Neistat, associate program and emergencies director at Human Rights Watch. "The Syrian Air Force knows very well that using cluster bombs and raining down missiles and bombs indiscriminately on urban areas violates the laws of war."