Iran has released 10 US sailors held for entering its territorial waters, in an incident that tested newly improved US-Iranian ties.

The US expressed its gratitude to Iran for swiftly resolving the issue.

The sailors were detained on Tuesday when one of their two vessels broke down while training in the Gulf.

It comes at a sensitive time, amid reports that a deal on Iran's nuclear activities is just days away from implementation.

Iranian state media said the sailors were released into international waters after they apologised. But White House officials said the US administration had not issued an apology to Iran.

The incursion was "unintentional", a statement from the Iranian Revolutionary Guards quoted by state media said.

General Ali Fadavi of the Revolutionary Guard said the sailors were not spying

Earlier, the naval commander of the Guards, Gen Ali Fadavi, said investigations found a navigational failure was to blame.

"We have concluded that passage of Americans in our territorial waters was not a hostile passage or for espionage or similar acts," he told Iranian television.

The US is investigating how the sailors entered Iranian waters.

US Secretary of State John Kerry said he wanted to "express my gratitude to Iranian authorities for their co-operation in swiftly resolving this matter".

"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," Mr Kerry said in a statement.(BBC)