Ten U.S. Navy sailors taken into custody by Iran after their two small boats drifted into Iranian waters were expected to be released as early as Wednesday morning local time.

American officials have said Tehran assured them that the crew and vessels would be returned safely and promptly. Meanwhile, the Navy chief of the powerful Iranian Revolutionary Guard said Wednesday that the American boats showed "unprofessional acts" for 40 minutes before being picked up by Iranian forces after entering the country's territorial waters.

The navy chief, Gen. Ali Fadavi confirmed on state TV Wednesday that the Guard was holding the crew and their two small boats. The detainees included nine men and one woman, who were being held overnight at an Iranian base on Farsi Island in the Persian Gulf. They were expected to be transferred Wednesday to a U.S. ship in the region.

Referring to the two small boats that had drifted into Iranian waters after experiencing mechanical problems, Fadavi also said the Guard will carry out orders from the country's top leadership over the case, suggesting the Americans could be freed soon.

Pentagon spokesman Peter Cook told The Associated Press that the Riverine boats were moving between Kuwait and Bahrain when the U.S. lost contact with them.

U.S. officials said that the incident happened near Farsi Island in the middle of the Gulf. They said some type of mechanical trouble with one of the boats caused them to drift into Iranian territorial waters near the island, and they were picked up by Iran.

The semi-official Iranian news agency, FARS, said earlier that the Iranian Revolutionary Guard's navy had detained 10 foreign forces, believed to be Americans, and said the sailors were trespassing in Iranian waters.

"We have been in contact with Iran and have received assurances that the crew and the vessels will be returned promptly," Cook said late Tuesday, U.S. time.

The incident came amid heightened tensions with Iran, and only hours before President Barack Obama gave his final State of the Union address to Congress and the public. It set off a dramatic series of calls and meetings as U.S. officials tried to determine the exact status of the crew and reach out to Iranian leaders.

Secretary of State John Kerry, who forged a personal relationship with Iranian Foreign Minister Mohammed Javad Zarif through three years of nuclear negotiations, called Zarif immediately on learning of the incident, according to a senior U.S. official. Kerry "personally engaged with Zarif on this issue to try to get to this outcome," the official said.

Kerry learned of the incident around 12:30 p.m. EST as he and Defense Secretary Ash Carter were meeting their Filipino counterparts at the State Department, the official said.

Officials said the sailors were part of Riverine Squadron 1 based in San Diego and were deployed to the U.S. Navy's 5th Fleet in Bahrain. When the U.S. lost contact with the boats, ships attached to the USS Harry S Truman aircraft carrier strike group began searching the area, along with aircraft flying off the Truman.

Officials said a radio signal from one of the boats showed that they were on Farsi Island, setting off efforts to contact the Iranians. The Riverine boats were not part of the carrier strike group, and were on a training mission as they traveled between Kuwait and Bahrain, officials said.

The Riverine boats are not considered high-tech and don't contain any sensitive equipment, so there were no concerns about the Iranians gaining access to the crafts.

The officials were not authorized to discuss the sensitive incident publicly so spoke on condition of anonymity.

The incident came on the heels of an incident in late December when Iran launched a rocket test near U.S. warships and boats passing through the Strait of Hormuz.

Meanwhile, Iran was expected to satisfy the terms of last summer's nuclear deal in just days. Once the U.N. nuclear agency confirms Iran's actions to roll back its program, the United States and other Western powers are obliged to suspend wide-ranging oil, trade and financial sanctions on Tehran. Kerry recently said the deal's implementation was "days away."

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Associated Press writers Bradley Klapper and Richard Lardner in Washington and Nasser Karimi and Ali Akbar in Tehran contributed to this report.