Vilified by accusations of using a chemical bomb, Syria’s president intensified his counterpropaganda campaign on Thursday, suggesting that child actors had staged death scenes to malign him and that American warplanes had bombed a terrorist warehouse full of poison gases, killing hundreds of people.

In his first interview since an April 4 attack on the northern town of Khan Sheikhoun that killed over 80 people, sickened hundreds and outraged the world, President Bashar al-Assad of Syria not only repeated the government’s denials of responsibility but contended without evidence that the episode had been fabricated as a pretext for an American retaliatory missile strike.

“We don’t know whether those dead children were killed in Khan Sheikhoun,” Mr. Assad told Agence France-Presse in the television interview from Damascus, which was recorded on Wednesday. “Were they dead at all?”

The decision by the increasingly isolated Syrian president to give an interview to a Western news organization appeared to reflect a calculation that his best option, even in the face of incriminating evidence, was to repeatedly deny responsibility for the attack, one of the worst in the six-year-old Syrian war.