Syria's government and rebels have each accused the other of a poison gas attack on a village near Aleppo.
The White House says it has no evidence to back up Syrian President Bashar Assad's claim the rebels have used chemical weapons. White House spokesman Jay Carney says the US is looking carefully at allegations that both sides are using them, but says he's sceptical of any claims made by the Syrian regime.
The government said 25 people were killed and ally Russia back its claim that the rebels were to blame.
If confirmed, it would be the first known use of chemical weapons in the two-year-old civil war and a glimpse of one of the nightmare scenarios for the conflict.
One of the international community's top concerns since fighting began is that Syria's vast arsenal of chemical weapons could be used by one side or the other or could fall into the hands of foreign jihadist fighters among the rebels or the Lebanese militant group Hezbollah, which is allied with the regime.
The accusations emerged only a few hours after the opposition to president Bashar Assad elected a prime minister to head an interim government that would rule areas seized by rebel forces from the regime.
The Syrian regime said at least 25 people were killed and 86 wounded, some in critical condition, in the missile attack on the village of Khan al-Assal.
State-run news agency SANA published pictures showing casualties, including children, on stretchers in what appears to be a hospital ward.
Information minister Omran al-Zoubi called it the "first act" of the newly announced opposition interim government.
Rebels quickly denied the report and accused regime forces of firing the chemical weapon.
But one US source said there was no evidence that either side used chemical weapons, adding that the Organisation for the Prohibition of Chemical Weapons also reported no independent information of their use.
The head of Syria's main opposition group, the Syrian National Council, said the group was still investigating the alleged attack.
Syria's policy has been not to confirm or deny if it has chemical weapons. But in July, a foreign ministry spokesman told said Syria would only use chemical or biological weapons in case of foreign attack, not against its own people.
The ministry then tried to blur the issue, saying it had never acknowledged having such weapons.
But the regime is believed to possess nerve agents as well as mustard gas. It also possesses Scud missiles capable of delivering them, and some activists said Tuesday's attack was with a Scud missile.
The regime claimed the missile containing "poisonous gases" was fired from Nairab district in Aleppo into Khan al-Assal.
The reported attack was in an area just west of the city of Aleppo that had seen fierce fighting for weeks before rebels took over a sprawling government complex there last month.
Russia's Foreign Ministry said rebels detonated a weapon containing an unidentified chemical agent, calling it an "extremely dangerous" development.
It said that evidence of rebels getting hold of weapons of mass destruction had taken the confrontation to a new stage.