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Jan. 12, 2016, 9:15 PM GMT / Updated Jan. 12, 2016, 11:35 PM GMT By Jim Miklaszewski and Courtney Kube

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 flashpoint 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.

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Following reports of the incident, Secretary of State John Kerry spoke with Iranian officials in Tehran by phone, attempting to secure the sailors' release.