United States sailors detained by Iran have been released, Iran's Revolutionary Guards said on Wednesday.

Iran was holding 10 U.S. Navy sailors and their two small Navy boats after the boats had mechanical problems and drifted into Iranian waters, but American officials had received assurances from Tehran that they will be returned safely and promptly.

Pentagon spokesman Peter Cook told The Associated Press on Tuesday that the riverine boats were moving between Kuwait and Bahrain when the U.S. lost contact with them.

U.S. officials said that the incident happened near Farsi Island, situated in the Persian Gulf. They said that some type of mechanical trouble with one of the boats caused them to run aground and they were picked up by Iran. The sailors were in Iranian custody on Farsi Island at least for some time, but it's not certain where they are now.

The semi-official Iranian news agency, FARS, said the Iranian Revolutionary Guard's navy has detained 10 foreign forces, believed to be Americans, and said the sailors were trespassing in Iranian waters.

"We have been in contact with Iran and have received assurances that the crew and the vessels will be returned promptly," Cook had said.

The incident comes amid heightened tensions with Iran, and only hours before President Barack Obama is set to deliver his final State of the Union address to Congress and the public. It set off a dramatic series of calls and meetings as U.S. officials tried to determine the exact status of the crew and reach out to Iranian leaders.

Secretary of State John Kerry, who has forged a personal relationship with Iranian Foreign Minister Mohammed Javad Zarif through three years of nuclear negotiations, called Zarif immediately on learning of the incident, according to a senior U.S. official. Kerry "personally engaged with Zarif on this issue to try to get to this outcome," the official said.

Kerry learned of the incident around 12:30 p.m. EST as he and Defense Secretary Ash Carter were meeting their Filipino counterparts at the State Department, the official said.

This comes on the heels of an incident in late December when Iran launched a rocket test near U.S. warships and boats passing through the Strait of Hormuz.

The officials were not authorized to discuss the sensitive incident publicly so spoke on condition of anonymity.

The incident also takes place just days before Iran is expected to satisfy the terms of last summer's nuclear deal. Once the U.N. nuclear agency confirms Iran's actions to roll back its program, the United States and other Western powers are obliged to suspend wide-ranging oil, trade and financial sanctions on Tehran. Kerry recently said the deal's implementation was "days away."