Iran is ratcheting up pressure on the U.N. agency responsible for overseeing the country’s nuclear program, accusing its inspectors of engaging in spying and sabotage and threatening to restrict U.N. access to Iranian nuclear facilities.

So strident has been Iran’s criticism of the International Atomic Energy Agency in recent weeks that some Western officials fear that the country is preparing to officially downgrade its cooperation with the nuclear watchdog. The Vienna-based agency is the only international body allowed to routinely visit Iran’s most sensitive nuclear installations.