Iran's Revolutionary Guards seized the tankers in the Gulf days after Athens confirmed it would deliver Iranian oil it had seized from the Russian tanker Pegas to Washington.

Greece swiftly accused Iran of "piracy". Its foreign ministry said Iranian helicopters had landed commandos on the two tankers.

The 49 crew members of Delta Poseidon and Prudent Warrior, including ten Greek sailors and one Cypriot, were held hostage at sea in the Persian Gulf for almost 100 days.

In a statement on Sunday, the Panhellenic Union of Merchant Navy Sailors said that during the recent visit of a Greek delegation to Iran, the Iranian side took a significant step back and agreed to the replacement of the two crews, Ert tv said.

The release of the sailors is expected to begin on Monday with the crew members of the Prudent Warrior ship while the members of the "Delta Poseidon" will follow.

Tehran allowed the release the crews after the 100,000 tons of oil originally seized by the Greek authorities at the request of the US was returned to the tanks of the Iranian-owned ship Lana late August.