Iranian military forces seized two U.S. Navy boats Tuesday and are holding them in custody on Iran's Farsi Island in the middle of the Persian Gulf, senior U.S. officials reported.

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 their boat experienced mechanical failure and drifted into Iranian-claimed waters and were detained by the Iranian Coast Guard, officials added. The crew was described as being safe.

One senior official stated the Iranians understand it was a mistake and have agreed to release the Americans in international waters within hours.

Sen. Barbara Boxer, D-Calif., said 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.