WASHINGTON — Iran detained 10 sailors aboard two small U.S. Navy boats on Tuesday, but assured U.S. officials the crew and vessels would be promptly returned.The two riverine boats were en route from Kuwait to Bahrain when the military lost contact with the vessels, a U.S. military official told USA TODAY. The U.S. is communicating with Iranian officials, who gave assurances of the sailors’ safety, the official said.But the semi-official Iranian news agency FARS reported that Iranian Revolutionary Guards arrested 10 U.S. troops for trespassing in Iranian waters. Nine men and one woman were detained. The vessels carried three .50-caliber machine guns, the agency reported.Secretary of State John Kerry spoke to his Iranian counterpart, Foreign Minister Mohammed Javad Zarif, who promised the two ships would be allowed to continue their voyage, likely at dawn Wednesday local time, according to a senior U.S. official.The two officials spoke on condition of anonymity because they were not authorized to discuss the incident publicly.The boats drifted into Iranian coastal waters in the vicinity of Farsi Island in the north Persian Gulf, apparently after one experienced mechanical problems and the other attempted to render aid. It was not clear how far into Iranian waters they traveled, but the move was unintentional, the senior official said.White House Deputy National Security Adviser Ben Rhodes said the administration was aware of the incident. “We are working to resolve the situation such that obviously the U.S. personnel are returned to their normal deployment.”The incident could be a key test for Iran-U.S. relations following a nuclear deal in which the United States and other world powers agreed to lift international sanctions in return for Iran reducing its nuclear program. Relief from the sanctions could begin as early as this week. Kerry and Zarif developed a close relationship during the long negotiations.