BEIRUT, Lebanon — The head of the United Nations’s nuclear watchdog agency surprised diplomats and arms control experts yesterday by announcing that inspectors would visit Syria for two days to try to clear up the mystery of an alleged nuclear site destroyed in an Israeli air strike last year.

The director-general of the International Atomic Energy Agency, Mohamed ElBaradei, told his board of governors in Vienna, Austria, that an inspection team would travel to Syria on June 22 to investigate the site. Mr. ElBaradei also voiced frustration about his organization’s probe of Iran’s nuclear program, which that nation insists is meant for peaceful electricity generation and Western officials believe masks a drive to produce weapons of mass destruction.

Mr. ElBaradei said it was “regrettable” that his agency had not made “the progress we had hoped for” regarding documents suggesting Iran was working on missile designs, uranium experiments, and explosives testing consistent with a nuclear weapons program. He also strongly criticized the country’s leadership.

“Iran has not yet agreed to implement all the transparency measures required to clarify this cluster of allegations and questions,” he told the IAEA governing board.

Israeli warplanes struck the disputed Syrian site in September in what many analysts considered a warning to Iran about its willingness to use force to prevent regional rivals from obtaining the capability to produce nuclear weapons. Israel is believed to have an arsenal of more than 300 nuclear warheads.