Ten US Navy sailors and their two small boats that drifted into Iranian waters after experiencing mechanical problems will be returned safely and quickly, Tehran has said.
The nine men and one woman were being held overnight at an Iranian base on Farsi Island in the Persian Gulf, and were expected to be transferred to a US ship in the region today.
Officials said they believe the US had spoken to one of the crew and all 10 were fine and uninjured.
Pentagon spokesman Peter Cook said the Riverine boats were moving between Kuwait and Bahrain when the US lost contact with them.
American officials said that the incident happened near Farsi Island in the middle of the Gulf. They said some type of mechanical trouble with one of the boats caused them to drift into Iranian territorial waters near the island and they were picked up by Iran.
The semi-official Iranian news agency FARS said the Iranian Revolutionary Guard’s navy detained 10 foreign forces, believed to be Americans, and 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,” Mr Cook said.
The incident came amid heightened tensions with Iran, and only hours before President Barack Obama delivered his final State of the Union address to Congress and the public.
It set off a dramatic series of calls and meetings as US officials tried to determine the exact status of the crew and reach out to Iranian leaders.
Secretary of state John Kerry, who forged a personal relationship with Iranian foreign minister Mohammed Javad Zarif through three years of nuclear negotiations, called Mr Zarif immediately on learning of the incident, according to a senior US official.
He “personally engaged with Zarif on this issue to try to get to this outcome”, the official said.
Mr Kerry learned of the incident as he and defence secretary Ash Carter were meeting their Filipino counterparts at the US State Department.
Officials said the sailors were part of Riverine Squadron 1 based in San Diego, California, and were deployed to the US Navy’s 5th Fleet in Bahrain.
When the US lost contact with the boats, ships attached to the USS Harry S Truman aircraft carrier strike group began searching the area, along with aircraft.
A radio signal from one of the boats showed that they were on Farsi Island, setting off efforts to contact the Iranians. The Riverine boats were not part of the carrier strike group and were on a training mission as they travelled between Kuwait and Bahrain, officials said.
The Riverine boats are not considered high-tech and do not contain any sensitive equipment, so there were no concerns about the Iranians gaining access to the crafts.
Republican politicians seized on the incident as further evidence that Iran was not to be trusted.
The House of Representatives is due to vote today on Republican-backed legislation that would give Congress greater oversight of the landmark Iran nuclear agreement, which party members have derided as a victory for Tehran.
Republican senator Kelly Ayotte said it was “unthinkable that the administration would lift sanctions and permit Iran to receive billions of dollars in sanctions relief under the nuclear agreement, even as the regime brazenly violates its international obligations and rushes to develop the ballistic missile capability to deliver a potential nuclear weapon to the United States”.
The incident came on the heels of an incident in late December when Iran launched a rocket test near US warships and boats passing through the Strait of Hormuz.
Meanwhile, Iran is expected to satisfy the terms of last summer’s nuclear deal in just days.
Once the United Nations nuclear agency confirms Iran’s actions to roll back its programme, the United States and other Western powers are obliged to suspend wide-ranging oil, trade and financial sanctions on Tehran. Mr Kerry recently said the deal’s implementation was “days away”.