BEIRUT, Lebanon -- The Syrian government and Syrian rebels traded accusations about a lethal attack in the northern province of Aleppo on Tuesday, in which each side in the country's two-year-old conflict said the other had used chemical weapons.
But neither side presented clear documentation, and two American officials said there was no evidence to suggest that any chemical weapons had been used. A Defense Department official said the claim should be treated with caution, if not outright skepticism.
The first report came from the Syrian state news agency, SANA, which reported that terrorists, its term for armed rebels, had fired a rocket "containing chemical materials" on Tuesday into the Khan al-Assal area of Aleppo province, killing 16 people and wounding 86. It later raised the death toll to 25.
The news agency displayed photographs of what it said were the victims, but there was no indication in the photographs that they had suffered a chemical attack, such as burns or skin discoloration or quarantine measures.
A senior rebel commander and spokesman, Qassem Saadeddine, later accused the government of using chemical weapons in the attack, citing reports of breathing difficulties and bluish skin among victims, but admitted the reports were secondhand and he could provide no documentation.
Another rebel commander, Abdul Jabbar al Okaidi, head of the rebel military council in Aleppo, said in a telephone interview that he had witnessed the attack, describing it as an errant strike on a government-controlled neighborhood, by Syrian warplanes flying at high altitude. He said the explosions from the attack emitted what he described as a gas that appeared to cause suffocation, and that some victims had been treated in a rebel field hospital.
The commander ridiculed government assertions that the rebels had chemical weapons. "We don't even have ammunition for our Kalashnikovs," he said.
Each side in Syria's conflict has an incentive to accuse the other of using chemical weapons. President Obama has said that a chemical attack by President Bashar al-Assad's government would cross a "red line" that could prompt military intervention against him by the United States.
And the Syrian government seeks to portray its opponents as extremists who are a threat to regional stability. Israel has said it would intervene to prevent chemical weapons from falling into the hands of either the rebels or Hezbollah, the Lebanese militant group allied with the Syrian government. Use or seizure of chemical weapons by rebel forces would embarrass the United States, particularly now, as President Obama has declared he will not oppose allied efforts to provide them with military aid.
Rebel factions have accused the government of using chemical weapons many times, with no confirmed cases. The term "chemical weapons" has sometimes appeared to be used loosely to include not just deadly nerve agents like sarin gas but also tear gas and other nonlethal irritants used for crowd control.
The Foreign Ministry of Russia, Mr. Assad's most powerful international backer, indicated that it was taking the government's claim seriously, calling the alleged use of chemical weapons by the opposition an "extremely dangerous development" and a new reason to refocus energy on finding a political solution to the conflict.
A Syrian official told state television that the Aleppo attack would be reported to human rights organizations and to countries supporting the rebels.
A Reuters photographer was quoted in a report by the news agency as saying that he had visited victims in Aleppo hospitals, and that they were suffering breathing problems.
"I saw mostly women and children," said the photographer, who Reuters said it could not identify by name out of concern for his safety. "They said that people were suffocating in the streets and the air smelt strongly of chlorine."
Rebels have long tried, without success, to overrun a weapons plant near Safira, in Aleppo province, where chemical weapons are believed to be stored. The government alleged in December that rebels had plundered supplies of chlorine gas, but the Syrian government's stores are believed by American officials to consist of other types of chemical weapons.
An Obama administration official, speaking on condition of anonymity, said the White House had "no information suggesting opposition groups have chemical weapons capability."
Some American officials worried aloud whether the Syrian government was accusing rebels of using the weapons to prepare cover for its own future use of them. Anti-government activists suggested that the government might have concocted the chemical attack story to cover up an incident in which it accidentally fired a Scud missile on a government-held area.
The Syrian Observatory for Human Rights, a Britain-based watchdog group, said witnesses had heard over walkie-talkies that 26 people were killed, including 16 government soldiers and 10 civilians, after a rocket landed on Khan al-Assal. Activists said the government had tried to hit the police academy there, which had recently been taken by rebel forces, with a Scud missile, but it accidentally fell on a government-controlled area instead.
In Washington, the White House cast doubt on claims that the opposition had used chemical weapons, and said it was evaluating the possibility that the government had used them.
"We're looking carefully at allegations of C.W. use, chemical weapons use," said Jay Carney, the White House press secretary.
He said the administration was "deeply skeptical" of the assertions by President Assad's government that the opposition had mounted a chemical attack.
Likewise at the State Department, a spokeswoman dismissed the Syrian government's claim as an effort to distract from its use of long-range Scud missiles against civilians. The spokeswoman, Victoria Nuland, said the United States was looking into rebel claims that the government had used chemical weapons and tried to blame its opponents.
Another U.S. official, speaking on condition of anonymity, said that officials do not believe a chemical attack took place, but are "nervous" about what the Syrian government may be thinking.
At the United Nations, diplomats reacted to the unverified reports with steep caution, although they did not totally discount the possibility. Sir Mark Lyall Grant, the ambassador to Britain, told reporters that "clearly if chemical weapons were used then that would be abhorrent and it would require a serious response from the international community." Australia's foreign minister, Bob Carr, who was visiting, said a chemical weapons attack "would be a shocking precedent if it is the case."
Reporting was contributed by Isabel Kershner from Ramallah, West Bank, Peter Baker, Michael R. Gordon and Eric Schmitt from Washington, Hwaida Saad from Beirut and Rick Gladstone from New York.world
This article originally appeared in The New York Times.