Iran has reportedly released the 49 crew members of two Greek tankers seized by its Revolutionary Guards in May after confiscation of Iranian oil by the US from a tanker off the Greek coast.

A Greek shipping ministry source said on Monday that "a very serious effort has been under way for the return of the Greek crews the soonest possible," without elaborating. Iran had said the crew were not detained and are on board their vessels.

The Greek union of commercial ships' seafarers said Sunday that Iran has agreed to release the crews so they will be replaced, allowing their return to their countries of origin as of Monday. It was not clear whether the two tankers, the MT Prudent Warrior and Delta Poseidon, would be released, a statement on the union’s website added.

Iran's foreign ministry spokesperson Nasser Kanaani did not confirm the information. "We have no confirmation at this moment about a release," he said early Monday, expressing hope that "there would be positive developments in the future".

Tehran allowed the release of 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.

The Iranian-flagged tanker Lana, formerly Pegas, was seized by Greece in April and was held for months, and the US had confiscated part of its oil cargo due to sanctions. The Iranian Revolutionary Guards' navy seized the two vessels during helicopter-launched raids in the Persian Gulf on May 27, a week after Washington chartered a vessel to transfer the Iranian oil to the US.