WASHINGTON — Two US Navy patrol boats and their crews were seized by the Iranian authorities in the Persian Gulf on Tuesday during what a Tehran news agency alleged was “snooping.’’

But the Pentagon and the State Department said that one of the boats had experienced mechanical problems while en route from Kuwait to Bahrain on a routine mission. Administration officials said that the military had lost contact with the boats before they strayed into Iranian territorial waters. They said they had received assurances from Iran that the 10 sailors would be returned soon, perhaps on Wednesday.

The semiofficial Fars news agency in Iran said that the boats had illegally traveled more than a mile into Iranian waters near Farsi Island, the site of a major Iranian naval base. It said that members of the Islamic Revolutionary Guards navy had confiscated GPS equipment, which would “prove that the American ships where ‘snooping’ around in Iranian waters.’’

The waters where the boats were sailing are a frequent location for intelligence collection by the United States, Iran, and many gulf countries. The US and Iranian navies encounter each other frequently there.

The US sailors were aboard two riverine patrol boats — 38-foot, high-speed boats used to patrol rivers and coastal waters. One official said the vessels failed to make a scheduled rendezvous with a larger ship to refuel.

The detention of the sailors comes at a particularly delicate moment in the tense American-Iranian relationship, a few days before the formal implementation of a nuclear deal in which the United States is supposed to unfreeze about $100 billion in Iranian assets.

That step is to be made after international nuclear inspectors verify Iran has shipped 98 percent of its nuclear fuel out of the country, removed centrifuges, and taken a large plutonium reactor permanently offline.

New York Times