The Islamic Revolutionary Guard Corps (IRGC) said it seized two cargo ships attempting to cross the Strait of Hormuz and directed them to Iranian territorial waters.
The IRGC identified one ship as MSC Francesca, which it said belonged to Israel, and the other as Epaminondas, which was accused of tampering with navigation systems and jeopardising maritime security.
Iran has warned against any action that violates its regulations in the strait, which accounts for a fifth of global oil and gas exports.
The UK Maritime Trade Operations Centre said one ship was attacked by an IRGC gunboat without warning, while Iran's state-run news agencies reported a third vessel, Euphoria, had become stranded on the Iranian coast.