Iran detained 10 US sailors after two small patrol boats reportedly drifted into Iranian waters in the Gulf, US officials say.
They say the Riverine boats apparently developed mechanical problems and were taken to Iran's Farsi Island.
Tehran says the crew and the boats "will be returned promptly", Pentagon spokesman Peter Cook told the Associated Press news agency.
The sailors were "snooping" in Iranian waters, Iran's Fars news agency says.
It says the sailors - nine men and a woman - were held by the Iranian Revolutionary Guard.
Kerry 'personally engaged'
"We lost contact with two small US naval craft en route from Kuwait to Bahrain," a senior US administration official said.
Tuesday's incident happened near Farsi Island in the middle of the Gulf.
The sailors are likely to be released on Wednesday morning, US officials said.
After the incident, US Secretary of State John Kerry immediately called Iranian Foreign Minister Javad Zarif to begin negotiations.
An unnamed official told the Associated Press that Mr Kerry "personally engaged with Zarif on this issue to try to get to this outcome".
Mr Kerry and Mr Zarif developed a personal relationship throughout three years of negotiating a nuclear deal.
The conservative Tasnim news agency reported that the American boats were equipped with machine guns.
"Frequent calls by US officials urging Tehran to free the detainees continues," reads the Tasnim report.
The Revolutionary Guard has aggressively protected Iranian sea borders in the past.
Fifteen British sailors and marines were held for 13 days in 2007 after they were captured in disputed area between Iran and Iraq.
Tensions between Iran and the US remain despite the breakthrough nuclear deal.
In December, Iran's navy conducted rocket tests near US warships and other commercial vessels in the Strait of Hormuz.
The tests were "highly provocative", a US military commander said at the time.