The Islamic Revolutionary Guard Corps seized the ships in the Gulf days after Greece said it would deliver Iranian oil it had taken from a Russian tanker to the United States.

The Russian ship, the Pegas, had been detained by Greece at the request of the United States, which has imposed crippling sanctions on Iran, particularly on its oil exports.

"The (Greek) ship's crew has been replaced according to international shipping protocols, which is done by the shipping companies themselves," Iran's foreign ministry spokesman Nasser Kanani said.

"The previous crew left the tanker under usual arrangements," he was quoted as saying by ISNA news agency, without elaborating.

The 49 crew members of Delta Poseidon and Prudent Warrior, including 10 Greek sailors and one Cypriot, were held in the Gulf for almost 100 days.

Greece's ERT state-run broadcaster said at the weekend that Iran had given the green light for the release of the crews from the two Greek tankers, which had been expected to begin on Monday.

Kanani said on Wednesday that "they had not been arrested and imprisoned in Iran".