At the Pentagon, spokesman Captain John Kirby said the crew of 15 pirates, all believed to be Somalis, were now being detained aboard the Stennis.

"They were held hostage, with limited rations, and we believe were forced against their will to assist the pirates with other piracy operations," he said.

"The Al Molai had been taken over by pirates for roughly the last 40-45 days," Josh Schminky, a Navy criminal investigative service agent aboard the guided-missile destroyer USS Kidd, said in a statement.

The rescue operation took place on Thursday, when forces with the USS John C. Stennis carrier strike group received a distress call from the master of the Al Molai, an Iranian-flagged fishing vessel, who said he was being held captive by pirates.

WASHINGTON - The same US aircraft carrier group that Iran warned not to return to the Gulf has rescued 13 Iranians held hostage for weeks by pirates in the Arabian Sea, the Pentagon said on Friday.

The United States does not have formal diplomatic relations with Tehran, and the State Department said there had been no official communication with Iran about the rescue, which it described as a "humanitarian gesture."

State Department spokeswoman Victoria Nuland told a news briefing that the United States was reviewing options for prosecuting the pirates.

"We're consulting with international partners. You know, sadly, this is not a new thing. We have more than 1,000 pirates who've been picked up at sea who are under prosecution in some 20 countries. So this is always a question of where to send them and who will do the prosecution," she said.

Iran ratcheted up tensions earlier this week by threatening to take action if the Stennis returned to the Gulf after departing on December 27.

Army chief Major General Ataollah Salehi said on Tuesday: "I recommend and emphasize to the American carrier not to return to the Persian Gulf ... we are not in the habit of warning more than once."

Iran announced plans on Friday to hold new naval exercises in the Strait of Hormuz next month, the latest in a series of forceful gestures in the world's most important oil shipping lane.

It was not clear whether Iran's navy was aware of the rescue operation but the freed Iranian hostages, now on their way back home, had thanked the US crew, the Navy said.

"The captain of the Al Molai expressed his sincere gratitude that we came to assist them. He was afraid that without our help, they could have been there for months," said Schminky.