Iran's Revolutionary Guards said on Wednesday it was interrogating 10 American sailors it had detained a day earlier, and dismissed talk of their prompt release.

Iran detained the sailors who were aboard two US navy boats in the Gulf in an incident that rattled nerves days ahead of the expected implementation of a landmark nuclear accord.

"If, during the interrogation, we find out that they were on an intelligence gathering mission, we will treat them differently," Guards spokesman Ramazan Sharif said in an interview with Tasnim news agency.

Late on Tuesday, a US defense official had said plans were in place for Iran to return the sailors early on Wednesday to a US Navy vessel in international waters. But Sharif said that was speculation.

"What others say about the sailors' prompt release is their speculation, and I don't confirm or deny it," he told Tasnim.

IRGC Naval Commander Rear Admiral Ali Fadavi told state television in an interview that a US aircraft carrier in the Gulf had acted "provocatively and unprofessionally" for 40 minutes by carrying out air and sea manoeuvres after Iran arrested the American sailors.

He said the sailors were transferred to Farsi island, near where they had been seized.

Tehran had asked Washington for an apology for "violating" Iran's territorial waters, Fadavi added.

"Iran's Foreign Minister Mohammad Javad Zarif was in touch with US Secretary of State (John) Kerry.  Zarif took a firm stance as the sailors had violated Iran's territorial waters and asked the United States for an apology," Fadavi said.

Both US and Iranian officials have described the sailors, whose boats may have inadvertently drifted into Iranian waters, as safe and well-treated. US defense officials said nine men and one woman were aboard the two vessels seized.  - Reuters