The United States has found no evidence to support a Syrian government allegation that opposition rebels used chemical weapons in an attack this week, a US official said Thursday.

"We have no indication that chemical weapons were used," said the official, speaking on condition of anonymity.

The Syrian government and opposition rebels have traded accusations over the alleged attack.

Damascus accused opposition rebels of using chemical weapons in Khan al-Assal near Aleppo on Tuesday and said that more than 30 people died. The opposition said the government staged the attack and also used banned chemical weapons in another incident near Damascus.

The United Nations is launching an investigation into the Syrian government's allegation.

Western nations have repeatedly warned that the introduction of chemical weapons, regarded as weapons of mass destruction, could be a potential game-changer in how the conflict is viewed internationally.

US President Barack Obama said Syria's government would be risking a serious international response if it let the "genie out of the bottle" by using chemical weapons in the civil war.

Speaking on the issue during his trip to Israel on Wednesday, Obama said he was "deeply skeptical" that opposition forces had used such arms, noting that Syria's government had retained control of chemical weapon stocks.