The officials said it’s unclear whether the 10 American sailors aboard two small riverine vessels had strayed into Iranian territorial waters before they were captured.

The sailors 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 added. Iran’s coast guard took them into custody, although the crew has been described as being safe.

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 release the Americans in international waters within hours.

The Persian Gulf has been a flash point in the Middle East, and the incident comes on the same day President Barack Obama is set to give his annual State of the Union address.

Senior U.S. military commanders also 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.