A U.S. navy ship has rescued 13 Iranians being held hostage by pirates in the Arabian Sea, days after Iran warned U.S. vessels not to return to the Persian Gulf.

The U.S. Navy announced Friday that it detected the pirates' skiff alongside the Iranian fishing vessel, the Al Molai, on Thursday after receiving a distress call from the Iranian ship. The Navy said a team of military personnel from the destroyer USS Kidd boarded the Iranian vessel, or dhow, and detained 15 pirates who were holding the Iranians hostage.

The Navy said the Iranian vessel had been under the control of the pirates for more than a month and was being used as a headquarters for pirate operations. It said the hostages are believed to have been forced to help with piracies.

A U.S. Navy spokeswoman told VOA Friday that the U.S. naval team was aware before the rescue that the dhow was flying an Iranian flag. Lieutenant Rebecca Rebarich says vessels are “obligated” to assist any “distressed” sailors, regardless of nationality.

Rebarich says the head of the Iranian crew expressed his “sincere gratitude” to the U.S. Navy, saying he feared that without the U.S. assistance, his crew would have been held hostage for months.

The Navy said the Iranians were given food and medical care and were released wearing USS Kidd embroidered hats. Navy officials say the sailors are now on their way home and most likely not aware of the recent tensions between Tehran and Washington.

Iran has recently threatened to close the Strait of Hormuz, in the Persian Gulf, and warned the U.S. not to operate aircraft carriers in the area – something it routinely does. U.S. officials have made clear that operations will continue as usual in the Gulf.

U.S. State Department spokeswoman Victoria Nuland said Friday the U.S. has not communicated with Iran about the rescue that she called a “humanitarian gesture.”

“The very same ship and set of vessels that the Iranians protested on its last voyage through Hormuz, the John C. Stennis carrier strike group, just rescued this Iranian dhow from pirates.”

The pirates – believed to be Somalis – are in detention at sea on the U.S. aircraft carrier USS John C. Stennis.