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Iran has detained 10 United States sailors aboard two small patrol boats in the Gulf in an incident that rattled nerves days before the expected implementation of a landmark nuclear accord with Tehran.

A US defence official said plans were in place for Iran to return the sailors to a US Navy vessel in international waters early on Wednesday, Reuters news agency reported.

However, a Revolutionary Guards spokesman told Iran's Tasnim news agency that talk of the immediate release of the US sailors was speculation: "I cannot confirm or deny that."

The US sailors were being interrogated, the spokesman told Tasnim.

Al Jazeera's Rosiland Jordan, reporting from Washington DC, said the White House was "working as quickly as it can" to release the boats and the sailors.

She added that it was unclear when Iran would release the boats.

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

US defence officials said nine men and one woman were on the two vessels.

Josh Earnest, the White House spokesman, 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."

'Snooping around'

Official Iranian news outlets confirmed the detention, saying the the boats "were snooping around".

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

News of the incident came hours before President Barack Obama made his final State of the Union address to the US Congress before leaving office in January 2017.

Obama did not mention the crew's detention in the hour-long speech, but he did tout the nuclear deal with Tehran, saying "the world has avoided another war".

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

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

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

Formal implementation of the accord could begin in days, following steps Iran agreed to take to curb its nuclear activities.

The exact details of the incident still remain unclear, but US officials admitted both ships might have drifted inadvertently into Iranian waters.

In a statement, Iran's Islamic Revolutionary Guard Corps said France's Charles de Gaulle aircraft carrier was near the seized US boats.

Source: Al Jazeera and agencies