US Navy riverine patrol boats, of the kind detained by Iran on Tuesday. Wikimedia Commons Ten American sailors are reportedly being held by Iran after two US Navy riverine patrol boats drifted into Iranian waters after experiencing mechanical difficulties.

Iran had originally told the US that the sailors would be returned "promptly," but officials now say that the sailors will spend the night in Iran, according to a CNN report.

Plans are now apparently in place for Iran to return the sailors to the Navy on Wednesday morning, Gulf time, a US defense official told Reuters.

"Earlier today, we lost contact with two small US naval craft en route from Kuwait to Bahrain," a senior administration official told Business Insider.

The official continued: "We subsequently have been in communication with Iranian authorities, who have informed us of the safety and well-being of our personnel. We have received assurances the sailors will promptly be allowed to continue their journey."

White House Press Secretary Josh Earnest told CNN that "we've received assurances from the Iranians that our sailors are safe."

The boats apparently experienced mechanical difficulties and drifted into Iranian-claimed waters while the 10 sailors aboard were on a training mission, officials told NBC. Iran has now seized the 10 American sailors, who are reportedly being held at an Iranian Revolutionary Guard Corps base on Farsi Island in the Persian Gulf.

The semiofficial Fars news agency in Iran said that members of the hardline IRGC had confiscated GPS equipment from the boats, according to The New York Times. The news agency said the data from the equipment would "prove that the American ships [were] 'snooping' around in Iranian waters."

Google Earth/Amanda Macias/Business Insider

A senior US administration official told CNN's Jim Sciutto that there's nothing to indicate anything hostile on the part of Iran. Administration officials also reportedly said that releasing the sailors at night would be "unsafe."

Another US official, however, told CNN's Barbara Starr that the IRGC confiscated the sailors' communications equipment, which is hostile.

The Navy ships were reportedly near Iran's Farsi Island to refuel, according to Sciutto.

Sciutto said it's possible the ships ran out of fuel or accidentally wandered into Iranian waters.

If the ships did mistakenly travel into Iranian waters, however, the first course of action wouldn't typically be to detain the ships and their crews, Jonathan Schanzer, vice president for research at the Foundation for the Defense of Democracies, told Business Insider.

"The traditional response would be to issue some kind of warning rather than to detain the ship, and now ... they're going to be detaining 10 sailors overnight, which is obviously not insignificant," Schanzer said.

Iranian Supreme Leader Ayatollah Ali Khamenei. Thomson Reuters

Ben Rhodes, the White House's deputy national security adviser, told reporters that the US is "working to resolve the situation such that any US personnel are returned to their normal deployment."

A senior US official told The Associated Press that US Secretary of State John Kerry immediately called Iran's minister of foreign affairs, Javad Zarif, upon learning of the incident at around 12:30 p.m. EST.

Kerry "personally engaged with Zarif on this issue to try to get to this outcome," the official said.

The latest incident comes on the heels of Iran's rocket test in late December near US warships and boats passing through the Strait of Hormuz.

The incident also comes hours before US President Barack Obama is due to give his final State of the Union address before Congress.

This is not the first time Iran has detained Western navy sailors operating in or near Iranian waters.

In 2004, 15 British Royal Navy personnel from a training team based in southern Iraq were detained while delivering a boat from Umm Qsar to Basra, the Telegraph noted.

This post is being continuously updated. Click here to refresh.

The Associated Press and Amanda Macias contributed to this report.