Somali pirates have released a cargo ship they hijacked almost two years ago, with the crew "very relieved" to see their NATO rescuers, the military alliance said Tuesday.

The Panama-flagged MV Orna was released on Saturday and the HNLMS Rotterdam, the flagship of the NATO anti-piracy operation in the area, provided medical assistance, food and water to the freed crew, a statement said.

Pirates armed with rocket-propelled grenades hijacked the Orna, a cargo vessel from the United Emirates, in the Indian Ocean northeast of the Seychelles in December 2010.

NATO quoted the doctor on board the Rotterdam, identified only as Lieutenant Commander Rijkers, as saying "the crew was very relieved to see us.

"Fortunately, the crew are in relatively good physical health. By helping each other, they managed to find a way to cope with the difficult circumstances they lived in," he said.

The Rotterdam is on patrol as part of Operation Ocean Shield, NATO's contribution to the anti-piracy effort in the waters around the Horn of Africa, the statement said.

It gave no details of how or why the ship was released, or how many crew were on board or what happened to the pirates.

NATO agreed in March to extend Operation Ocean Shield for a further two years to 2014 to cope with pirate attacks which have fallen in recent months.