Iran's Revolutionary Guards have dismissed talk of the prompt release of 10 US sailors as "speculation", after their arrest in boats that reportedly entered Iranian territorial waters on Wednesday.

"What others say about the sailors' prompt release is their speculation, and I don't confirm or deny it," Guards' spokesman Ramazan Sharif said in an interview with Tasnim news agency, adding that the American sailors were being interrogated.

The Guards' naval commander, Rear Admiral Ali Fadavi, also told state television that a US aircraft carrier acted "provocatively and unprofessionally" for 40 minutes by carrying out manoeuvres in the Gulf after the arrest.

Earlier, a Pentagon official said plans were in place for Iran to return the sailors to a US Navy vessel in international waters later tonight.

US defence officials said nine men and one woman were aboard the two vessels, which lost contact with their base during a training exercise between Kuwait and Bahrain.

The incident has rattled nerves ahead of the expected implementation of a landmark nuclear accord with Tehran.

Admiral Fadavi said Tehran had asked the US for an apology after its sailors were arrested for "violating" its waters.

"Iran's Foreign Minister Mohammad Javad Zarif was in touch with US Secretary of State Kerry," he said.

Iran's elite Revolutionary Guard ride in their boat alongside an Iranian naval vessel (file image) ( AFP: IRNA )

"Zarif took a firm stance as the sailors had violated Iran's territorial waters and asked the United States for an apology."

Both US and Iranian officials described the sailors, whose boats may have inadvertently drifted into Iranian waters, as safe and well-treated.

White House spokesman Josh Earnest told CNN: "We have received assurances from the Iranians that our sailors are safe and that they will be allowed to continue their journey promptly."

Official Iranian news outlets confirmed the detention of the 10 Americans, hours before US President Barack Obama was made his final State of the Union address to the US Congress.

"The Revolutionary Guards naval forces seized the American boats two kilometres inside Iranian territorial waters while they were snooping around," Iran's semi-official Fars news agency said.

Officials from Iran and the US were negotiating to free the crew, Fars reported.

Facts around the ships' seizure remain unclear

Washington and Tehran, which have pursued a partial detente in recent years, both appeared eager not to let the incident escalate further.

Mr Obama has made the Iran nuclear accord a centrepiece of his foreign policy, and Republicans vying to succeed him have assailed him over the deal.

Iran, meanwhile, is eager for relief from punishing economic sanctions under the landmark nuclear accord it forged with six world powers last July.

Mr Zarif assured Mr Kerry that the sailors would be allowed to continue their journey promptly, another US official said.

The seizure of the 10 sailors nonetheless underscored the potential for clashes that could derail diplomacy, especially in the Gulf's tense and crowded shipping lanes.

Precisely what happened to the two US boats remained unclear.

A US official said mechanical issues may have disabled one of the boats, leading to a situation in which both ships drifted inadvertently into Iranian waters.

Reuters