"varying degrees of confidence" that the Syrian government has twice used chemical weapons in its civil war.
In the Syrian capital however, a government official said President Bashar Assad's military "did not and will not use chemical weapons even if it had them." He instead accused opposition forces of using them in a March attack on the village of Khan al-Assad outside of the northern city of Aleppo.
Both sides have accused each other of the deadly attack.
The official said the Syrian army had no need for using chemical weapons "because it is capable of reaching any area in Syria it wants" without them. He spoke to The Associated Press on condition of anonymity because he was not authorized to give official statements.
Sharif Shehadeh, a Syrian lawmaker, said the Syrian army "can win the war with traditional weapons" and has no need for chemical weapons.
Syria's official policy is not to confirm nor deny it has chemical weapons.
Shehadeh called the US claims "lies" and likened them to false accusations that Iraq possessed weapons of mass destruction - a claim US policymakers had used to justify the invasion of that country in 2003.
"What is being designed for Syria now is similar to what happened in Iraq when Colin Powell lied in the Security Council and said Iraq possesses weapons of mass destruction prior to the US invasion and occupation of that country," he said.
President Barack Obama has said that the use of chemical weapons would be a "red line" that could result in a significant military response. But the administration said on Thursday that the new revelation won't immediately change its stance on intervention.
On the streets of Damascus, the two-year old conflict dragged on Friday, with government troops pushing into two northern neighborhoods, triggering heavy fighting with rebels as they tried to advance under air and artillery support, activists said.
The drive was the latest in a dayslong offensive by government forces in and around the capital, an apparent bid to secure Assad's main stronghold against rebel challenges.
The Britain-based Syrian Observatory for Human Rights said the fighting between rebels and soldiers backed by pro-government militiamen was concentrated in the Jobar and Barzeh areas.
The Local Coordination Committees, another activist group, said troops also bombarded the nearby neighborhood of Qaboun with mortars and multiple rocket launchers.
State-run news agency SANA said troops killed five rebels in clashes near the main mosque in Jobar. It added that many other "terrorists," the term the government uses for rebels, were killed in the area and the nearby neighborhood of Zamalka.
The regime has largely kept the rebels at bay in Damascus, although opposition fighters control several suburbs of the capital from which they have threatened the heart of the city.
In March, government troops launched a campaign to repel the opposition's advances near the capital, deploying elite army units to the rebellious suburbs and pounding rebel positions with airstrikes.
The Observatory also reported clashes in the northern city of Aleppo, Syria's largest, between rebels and Kurdish gunmen in the contested Sheikh Maqsoud neighborhood. It also said there was fighting around the sprawling Abu Zuhour air base in the northwestern Idlib province.
Syria's conflict started with largely peaceful protests against Assad's regime in March 2011 but later degenerated into a civil war, which has left an estimated 70,000 dead.