Iran said on Saturday the crew of two Greek tankers seized by its Revolutionary Guards on Friday had not been detained and are on board their vessels.

Iranian forces seized two Greek tankers in the Persian Gulf on Friday, shortly after Tehran warned it would take "punitive action" against Athens over the confiscation of Iranian oil by the United States from a tanker held off the Greek coast.

"The crew of the two Greek tankers have not been arrested, and all crew members ... are in good health and are being protected, and provided with necessary services while on board, in accordance with international law," Iran's Ports and Maritime Organization said in a statement carried by state media.

The two vessels were stopped over unspecified "maritime violations", the body said.

Greece said on Friday an Iranian navy helicopter landed on Greek-flagged vessel Delta Poseidon in international waters and took the crew hostage. It said a similar incident took place on another Greek-flagged vessel near Iran, without naming the ship. Athens said both actions violated international law.

Greek authorities last month impounded the Iranian-flagged Pegas off Greece due to European Union sanctions. The United States later confiscated the Iranian oil cargo held onboard.

The Pegas and its Russian crew were later released, but the seizure inflamed tensions as Iran and world powers seek to revive a 2015 nuclear deal.

Separately, Nour News, affiliated to an Iranian state security body, said: "Iran will not remain passive in the face of any threat to its interests, and testing Iran's will is a strategic error that will entail heavy costs for the United States and its entourage."

Reporting by Reuters