The Syrian government and rebels accused each other Tuesday of firing a chemical weapon near the city of Aleppo, killing at least two dozen people in an attack that, if confirmed, would mark the first use of chemical arms and a major escalation in Syria’s two-year-old conflict.

Neither claim was independently confirmed Tuesday, nor was it clear what sort of weapon had been used in the attack. The United States, which has warned President Bashar al-Assad that a chemical attack would be a “red line” that would trigger a U.S. military response, expressed deep skepticism about the reports.

Syrian Information Minister Omran al-Zoubi said rebels fired “a rocket containing poison gases” at the town of Khan al-Assal, southwest of Aleppo, from a part of the city they control. The state-run Syrian Arab News Agency (SANA) said 25 people were killed and 86 injured in the attack. Rebels, in turn, accused forces loyal to Assad of firing a Scud missile containing deadly chemicals.

The Britain-based Syrian Observatory for Human Rights said 26 people died, including 16 soldiers. But the observatory’s director, who uses the pseudonym Rami Abdulrahman, said he could only “confirm that there was a rocket attack but not that any chemicals were used.”

In Washington, White House spokesman Jay Carney said the allegations were being studied. State Department spokeswoman Victoria Nuland characterized the Syrian claim as an attempt to discredit the opposition and said the United States has no evidence that rebels have chemical weapons capabilities.

A senior State Department official, speaking on the condition of anonymity because the reports were preliminary, said U.S. officials think the Syrian government has retained control over its chemical weapons supplies.

Rep. Mike Rogers (R-Mich.), chairman of the House Intelligence Committee, told CNN’s Wolf Blitzer on Tuesday that there is a “high probability” that Syria has used chemical weapons in the war but that final verification is needed.

Photos posted by SANA showed alleged victims in hospital beds flanked by medical staff in surgical masks. Footage aired on state television featured an interview with an elderly man wearing a face mask and a white bandage on his forehead. “They fired a missile, and it exploded with something like a powder,” the man said.

In the same state TV report, a doctor said the patients appeared to have been exposed to “phosphorus material or poisonous material,” which he said had led to “heavy vomiting and difficulty in breathing, almost appearing like extreme suffocation cases.”

Reuters news agency reported that one of its photographers visited victims in Aleppo hospitals who were suffering breathing problems and that people said they could smell chlorine after the attack.

Russia, an ally of the Assad government, charged that Syrian rebels had used a chemical weapon, an act that Moscow called alarming and dangerous.

In a statement, opposition activists from Khan al-Assal said the Syrian military had tried to target rebel positions with a missile but had hit a regime-controlled area instead.

Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

“This was a land-to-land rocket, which the opposition does not have. They do not have poison gases either,” said an activist who goes by the alias Majid Abdullah.

Opposition figures noted that the government has not previously reported that rebels have seized any chemical weapons.

The attack came hours after the Syrian opposition elected Ghassan Hitto, 50, a Syrian American information technology executive, as the prime minister of an interim government that would administer areas of Syria held by rebel forces.The election, at an opposition conference in Istanbul late Monday, could lead to closer links between the political opposition outside the country and the fighters and local officials governing rebel-held territory inside Syria.

William Branigin and Anne Gearan in Washington and Suzan Haidamous in Beirut contributed to this report.