Iranian military forces seized two U.S. Navy boats Tuesday and are detaining them on Iran's Farsi Island in the Persian Gulf, senior U.S. officials told NBC News.
The 10 American sailors aboard the small riverine vessels were on a training mission around noon ET when one of the boats may have experienced mechanical failure and drifted into Iranian-claimed waters, officials said. Iran's coast guard took them into custody.
The officials said it's unclear whether the sailors — nine men and one woman — had strayed into Iranian territorial waters before they were captured.
American naval ships routinely navigate the waters of the Persian Gulf near Iranian territory. The Gulf has been a flash point in the Middle East, and the incident comes on the same day President Barack Obama was set to give his annual State of the Union address.
"At this point, we're still monitoring the situation and it's not entirely clear what happened," White House spokesman Josh Earnest said on MSNBC, adding that Iranian authorities confirmed that the sailors are safe.
Earnest said there remained no exact time frame for when the sailors would be let go, but didn't expect a long delay. Officials told NBC News that the Americans aren't expected to be released until around daybreak Wednesday.
Following reports of the incident, Secretary of State John Kerry spoke with Iranian officials in Tehran by phone, attempting to secure the sailors' release.
One senior official told NBC News the Iranians understand it was a mistake and have agreed to let the Americans go in international waters.
Senior U.S. military commanders had criticized Iran last month for its "highly provocative" actions when it fired unguided rockets near U.S. ships, including the USS Harry S. Truman aircraft carrier.
Pentagon spokesman Peter Cook told The Associated Press that the boats were moving between Kuwait and Bahrain when U.S. officials lost contact with them. Farsi Island sits in the Persian Gulf in between Iran and Saudi Arabia, and is fortified by Iranian Revolutionary Guards.
"We have been in contact with Iran and have received assurances that the crew and the vessels will be returned promptly," Cook said.
Retired Army Gen. Barry McCaffrey, an NBC News military analyst, said the seizure of American boats is "very bad news" and highlights how the Navy may have failed to have surveillance on its vessels.
"We simply can't allow ground, air or naval units to be seized by a foreign hostile power," McCaffrey said on MSNBC. "I hope they're back real quickly. This is an affront to our military presence in the Gulf and will unsettle our allies in the region."
GOP presidential candidate Jeb Bush was among the first to attack Obama for appearing weak in front of Iran, which has been at odds with the U.S. over adelicate nuclear deal that was finally reached last year.
"No more bargaining. Obama's humiliatingly weak Iran policy is exposed again," Bush tweeted.
GOP rival Marco Rubio, during an interview on Fox News, called for Iran to release the sailors "immediately," saying the nation is "testing the boundaries of our resolve."
But Sen. Barbara Boxer, D-Calif., said on MSNBC that the pending release of the sailors shows how diplomacy between U.S. and Iranian officials "is working."
"Hopefully this will be resolved, and it won't be an issue that we're talking about in a couple of hours," she said.
Erik Ortiz, Alex Jaffe and The Associated Press contributed. This article first appeared on NBCNews.com.