Iran moved Wednesday to head off a potential crisis days before the expected implementation of its nuclear deal with world powers by releasing 10 US Navy soldiers it had detained in the Gulf.

A dramatic series of events started with the sailors -- nine men and a woman -- being taken into custody after their two patrol boats drifted into Iranian territory late on Tuesday.

US and Iranian officials scrambled to defuse the situation, which unfolded as the nuclear accord edged toward its final steps, with a top Iranian official saying the deal should be implemented by Sunday.

The detention of the sailors raised alarm in Washington but after informal talks with Tehran, Iran's Revolutionary Guards said they had been set free.

"Following their apology, they have been released to international waters in the Gulf," said a statement read out on state television, noting the sailors had not entered Iranian waters intentionally and had no "hostile intent".

Video footage showed the Navy personnel with their hands on their heads as they were apprehended. But other footage showed them eating a meal and drinking water, some smiling, while sitting on Persian rugs.

One sailor told Iranian state television that the crew had been treated well during their detention.

"It was a mistake and that was our fault and we apologise for our mistake," he said.

The Pentagon confirmed they had been freed and that there was no indication that the sailors had been harmed.

"The Navy will investigate the circumstances that led to the sailors' presence in Iran," it said.

- 'Broken navigation system' -

Admiral Ali Fadavi, the naval commander of the Guards, said an investigation established that "this trespassing was not hostile or for spying purposes".

Instead "a broken navigation system" had led them astray, he said.

US officials had said one or both of the boats experienced mechanical problems and they had been taken to Farsi Island, which lies roughly midway between Iran and Saudi Arabia in the Gulf and houses a base of the Guards, which has its own naval units.

Radio contact was lost with the two vessels -- riverine patrol boats under 65 feet (20 metres) in length -- while they were en route from Kuwait to Bahrain.

Washington and Tehran have no diplomatic relations, but US Secretary of State John Kerry called Iranian counterpart Mohammad Javad Zarif five times about the issue on Tuesday.

Kerry told him the sailors' release could be turned into a "good story" for both sides, according to a senior US official.

"That this issue was resolved peacefully and efficiently is a testament to the critical role diplomacy plays in keeping our country safe, secure, and strong," Kerry said in a statement.

Zarif said on Twitter: "Happy to see dialog and respect, not threats and impetuousness, swiftly resolved the #sailors episode. Let's learn from this latest example."

Kerry and Zarif developed a close working relationship during the nuclear talks, which concluded in July with a deal between Iran and the P5+1 powers of the United States, Britain, China, France, Russia and Germany.

- Iran 'testing the boundaries' -

The nuclear accord foresees Iran scaling back its activities to put an atomic bomb outside its reach in exchange for relief from crippling international sanctions.

Iran's deputy foreign minister Abbas Araghchi was quoted by Iranian media as saying that UN nuclear inspectors would issue a report on Friday that would be followed by announcement of the deal's implementation by Sunday.

Kerry, who has been criticised by President Barack Obama's opponents in the US congress as too soft on Tehran, last week said the agreement would be implemented "in the coming days".

Those rivals used the incident in the Gulf to hammer on this point, calling on Obama make a statement and warning Iran must release the sailors.

"Iran is testing the boundaries of this administration's resolve," Republican presidential hopeful Marco Rubio said.

The US House of Representatives on Wednesday passed a measure aimed at blocking the nuclear deal, but abruptly cancelled the vote because nearly one third of lawmakers were absent.

The Iran Terror Finance Transparency Act, which would bar the US administration from lifting certain sanctions against Iran, passed by a vote of 191 to 106, a low total by any measure in the 435-member chamber.

The vote is to be rescheduled for the week of January 25.

Iran's Guards often take a tough approach in what it considers the "Persian Gulf".

Relations with Washington were strained by claims last month that Iran fired rockets close to a US aircraft carrier in the Gulf.

Last year, Iranian patrol boats seized the Maersk Tigris, a cargo ship sailing under the Marshall Islands flag, which meant it was under US protection.

And in March 2007, Iranian patrols captured 15 British Royal Navy and Royal Marines personnel, interrogated them and held them for 13 days before releasing them.

burs-pg/dv

KERRY GROUP

A.P. MOELLER-MAERSK