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. Medical examiners in Turkey, where many of the Khan Sheikhoun victims were taken, have said that autopsies showed they had been attacked with sarin, a lethal nerve agent and a banned chemical weapon. The interview with Mr. Assad was broadcast as the Syrian government's news agency asserted without evidence that American warplanes had bombed what it called a chemical weapons cache possessed by Islamic State militants in Syria on Wednesday, leaving hundreds dead, including "a large number of civilians, due to suffocation caused by the inhalation of toxic materials."