Underwater pumping operations have got under way to remove some of the 500,000 gallons of fuel aboard the Costa Concordia, nearly a month after the cruise ship ran aground off Tuscany.

After nearly two weeks of delays because of rough seas and bad weather, the pumping got under way on the first of 15 tanks that are believed to hold around 84% of the fuel on board, Italy's civil protection department said.

Officials said it will take 28 consecutive days of pumping to empty the tanks. Dutch shipwreck salvage firm Smit is overseeing the operation, along with an Italian partner.

The fuel extraction process involves fixing valves on the underwater fuel tanks, one on top, one on bottom. Hoses are attached to the valves and as the oil - which must be warmed to make it less gooey - is sucked out of the upper hose, sea water is pumped in to fill the vacuum via the lower hose.

The Concordia slammed into a reef off the tiny island of Giglio after the captain deviated from the ship's planned course in an apparent stunt. Passengers have said the captain then delayed sounding the evacuation alarm until the ship had capsized so much that lifeboats on one side couldn't be lowered.

About 4,200 passengers and crew escaped, but 17 bodies have been found and another 15 people remain missing and presumed dead.

The captain, Francesco Schettino, is under house arrest, accused of manslaughter, causing a shipwreck and abandoning the ship before all the passengers had been evacuated. He has said the reef was not marked on his nautical charts.

During a memorial service for victims, Italy's president and the country's top bishop demanded justice and called for the truth of what transpired that night to come to light.

"Let the light of the Lord help bring about truth and justice, let wounds be healed and let trust and courage for the future be reinforced," Cardinal Angelo Bagnasco said in his homily.

President Giorgio Napolitano, who attended the service at Rome's St Mary of the Angels basilica, said he was sorry that what transpired "was the responsibility of Italy and Italians".