The head of the International Atomic Energy Agency said Thursday that a target destroyed by Israeli warplanes in the Syrian desert in 2007 was a covertly built nuclear reactor, countering assertions by Syria. The United States has said the target was a reactor, and previous reports by the nuclear watchdog had suggested that it could have been. At a news conference in Paris on Friday, the agency’s director, Yukiya Amano, said the facility was “a nuclear reactor under construction.” The agency later issued a statement saying it had not concluded “that the site was definitely a nuclear reactor,” suggesting Mr. Amano’s comments did not reflect official policy.