The Pentagon and the State Department said that one of the boats had experienced mechanical problems en route to Bahrain from Kuwait on a routine mission on Tuesday, and the Iranians appeared to have accepted that explanation.
The release was announced shortly before 10 a.m. on an Iranian state-run news channel, IRINN. “The detained U.S. Marines, after it was realized that their entry into Iran’s territorial waters was unintentional, and after the Marines apologized, were released into international waters in the Persian Gulf,” the channel reported, attributing the statement to the Islamic Revolutionary Guards Corps.
The United States Navy confirmed the release shortly thereafter. “Ten U.S. Navy Sailors safely returned to U.S. custody today, after departing Iran,” the United States Naval Forces Central Command in Bahrain said in a statement. “There are no indications that the sailors were harmed during their brief detention.”
The statement added that the Navy would “investigate the circumstances that led to the sailors’ presence in Iran.”
Administration officials said that the military had lost contact with the vessels before they strayed into Iranian waters.
Brig. Gen. Ramezan Sharif, a spokesman for the Revolutionary Guards, had said the sailors were held under good conditions and treated with “Islamic compassion.”
The semiofficial Fars news agency in Iran said that the boats had illegally traveled more than a mile into Iranian waters near Farsi Island, the site of a major Iranian naval base. It said that members of the Revolutionary Guards Navy had confiscated GPS equipment, which would “prove that the American ships were ‘trespassing’ around in Iranian waters.”
The waters in question are a frequent site of intelligence collection by the United States, Iran and many gulf countries. The American and Iranian Navies frequently encounter each other there.
The detention and release of the sailors comes at a particularly delicate moment in the tense American-Iranian relationship, just days before a nuclear deal is to be formally put in place, and under which the United States is to unfreeze about $100 billion in Iranian assets.
That step is to be made after international nuclear inspectors verify that Iran has shipped 98 percent of its nuclear fuel out of the country, has disabled and removed centrifuges, and has taken a large plutonium reactor permanently offline.
The American sailors were aboard two riverine patrol boats — 38-foot, high-speed boats that are used to patrol rivers and littoral waters. One official said the two vessels, which often patrol shallow waters near Bahrain, had failed to make a scheduled meeting with a larger ship to refuel.
Secretary of State John Kerry was notified of the seizing of the sailors while meeting with top Philippine officials and Defense Secretary Ashton B. Carter, an official said. Mr. Kerry broke off the meeting and called his Iranian counterpart, Mohammad Javad Zarif, with whom he had spent months negotiating the nuclear accord. As he walked into the House chamber Tuesday night for President Obama’s State of the Union address, Mr. Kerry said the sailors were “going to get out.”
Mr. Zarif’s ability to intervene in the matter was unclear. Many American and Middle Eastern officials say they believe that recent actions by the Iranian Navy against American forces in the gulf may be intended to embarrass Mr. Zarif and President Hassan Rouhani. The Revolutionary Guards was responsible for the military side of the nuclear program, and many of its senior officers have objected to the nuclear agreement.
American and European officials say that the nuclear accord should go into effect sometime next week. That is called “implementation day,” and it is crucial to Mr. Rouhani, who wants to demonstrate before parliamentary elections that he has succeeded in getting oil and financial sanctions lifted, and Iranian funds unfrozen.
Many Republicans in Congress have vowed to prevent that day from coming. Mr. Obama issued a veto threat on Monday against a House bill that would delay implementation day until the president can certify that Iran has reported all of its past work toward designing a nuclear weapon. International inspectors recently declared that Iran had a program “consistent” with weapons work through 2009, but that it had then ceased. Iran has always denied it ever sought a weapon.
Mr. Obama and Mr. Rouhani share the same problem: Their political opponents want to kill the nuclear deal. Both men are determined to see it through. The United States Treasury Department is expected to place some new sanctions on Iran for recent missile tests — which are not covered by the nuclear pact — but that effort has been delayed for reasons American officials will not discuss. A draft of the sanctions declaration was circulated on Capitol Hill just before the new year and quickly leaked.
In the skies and waters of the Persian Gulf, the Strait of Hormuz, the Arabian Sea and the Gulf of Aden, Iran and the United States constantly watch each other. American naval ships roam the waters along Iran’s 1,100-mile southern coastline, their radar trained on the shore and on Iranian ships leaving their harbors. Iranian fighter jets patrol the skies, keeping an eye on American combat planes that take off from an aircraft carrier in the Persian Gulf every time an Iranian jet comes close to their ship.
The Navy’s Fifth Fleet maintains a presence in the Persian Gulf, including the aircraft carrier, and it has had several episodes with Iran recently.
Two weeks ago, the Iranian Navy harassed an American carrier and a French frigate in the Strait of Hormuz, launching rockets that passed within 1,500 yards of the aircraft carrier Harry S. Truman.
Last year, an Iranian Navy frigate approached a ship in the Gulf of Aden where an American military helicopter had just landed and pointed a heavy machine gun at it for several minutes before turning around, all while an Iranian crew filmed the encounter. The Fifth Fleet, for its part, has its own videotape of the episode.
In 2007, the Revolutionary Guards Navy captured 15 British military service members and held them for 13 days, making a point of protecting its sea borders. A year later, the British Navy released a report saying that its vessels had been in an area with disputed borders between Iran and Iraq.