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.

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 by the United States.

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.