At least 26 people were killed by a rocket blast in the Syrian city of Aleppo, according to a human rights group, while both the Assad regime and rebels are pointing fingers at each other for the attack.
Both sides say chemical weapons were used, but the claims are being disputed.
White House spokesman Jay Carney said there is no evidence that Syrian rebels used chemical weapons. State Department spokeswoman Victoria Nuland added there is "no reason to believe the allegations" that chemical weapons had been used, but reiterated that use of such weapons by the Assad regime would constitute a "red line" for the U.S.
Rami Abdelrahman, head of the Syrian Observatory for Human Rights, told Reuters that 16 Syrian Army soldiers were killed in the explosion, and 10 others died in a local hospital. He did not elaborate whether they were soldiers or civilians.
Syrian information minister Omran al-Zoubi said the rocket, fired from Nairab district in Aleppo into Khan al-Assal village on Tuesday, contained "poisonous gases."
A Reuters photographer who had visited Aleppo hospitals said patients were suffering from breathing problems, and the air had a strong smell of chlorine.
Al-Zoubi claims the attack was the "first act" of the new Syrian opposition interim government announced in Istanbul, and said 86 people were wounded. He spoke to the pro-government Al-Ikhbariya TV station. Syria's state-run news agency, SANA, later said 25 were dead. SANA published pictures showing casualties, including children, on stretchers in what appears to be a hospital ward. None showed signs of physical injuries
The foreign ministry of Russia -- who has been the main ally of President Bashar Assad's regime since the start of the uprising -- backed the government's statements, saying rebels were the ones who used chemical weapons.
The ministry said the attack represented an "extremely dangerous" development in the two-year Syrian crisis. It said the rebels detonated a munition containing an unidentified chemical agent early Tuesday in the province of Aleppo, but didn't give further details.
However, Syrian rebels have denied the accusations and said regime forces were behind the attack.
"Fighting was raging in Khan al-Assal this morning and the regime's army hit the town with a long-range missile equipped with a chemical warhead," Louay al-Meqdad, a coordinator for the Free Syrian Army, told Reuters. "It also hit the area with conventional weapons from the air and with artillery.''
The regime has not said that rebels have been able to seize any chemical weapons, "so we assume that the opposition does not possess such weapons," said Mustafa Alani, an analyst with the Gulf Research center in Geneva.
"I would not rule out that the military would use chemical weapons and try to pin it on the rebels," Alani said.
Britain said use or proliferation of chemical weapons in Syria should warrant a serious response from the international community, Reuters reports.
In 2012, U.S. President Barack Obama said during an Aug. 20 press briefing that chemical weapons activity in Syria would be a "red line."
"We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized," he said.
Carney added on Tuesday that Obama was clear with his comments and " if Assad and those under his command make the mistake of using chemical weapons or fail to meet their obligations to secure them, then there will be consequences and they will be held accountable."
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.
Syria's policy has been not to confirm or deny if it has chemical weapons. But in July, then-Foreign Ministry spokesman Jihad Makdissi told a news conference that 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.
The Assad 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.
Meanwhile, the Syrian opposition's newly elected interim prime minister has ruled out dialogue with Assad's regime.
In a speech in Istanbul following his election Tuesday, Ghassan Hitto says "there is no place" for dialogue with Assad.
He also said all members of the regime who have committed crimes will be tried.
Hitto added that the interim government will be headquartered in rebel-held territories in northern Syria and urged international recognition.
At least 70,000 have died in Syria since the uprising against Assad began.
The Associated Press contributed to this report.