DPA

Operations to pump thousands of litres of potentially hazardous fuel from the tanks of the shipwrecked Costa Concordia have begun, after weeks of delay caused by bad weather.

The emptying of 15 of the Concordia's fuel tanks - where 84 per cent of its roughly 2400 tonnes of fuel are stored - would require 28 working days, weather conditions permitting, officials say.

The Concordia ran aground near Giglio, a small island off Italy's western coast, on January 13, after veering off its official course.

To date, 17 people have been certified dead from the disaster, while 15 remain unaccounted for.

The search for the missing people on parts of the ship lying under water was called off on January 31 due to safety concerns for scuba divers as rough water had caused the Concordia to shift slightly from its position.

The process of emptying the ship's tanks was originally scheduled to start soon after the end of the search for the missing people.

Earlier, a memorial mass attended by Italian President Giorgio Napolitano was celebrated in Rome for the victims of the shipwreck.

"It was a tragedy and I express regret for whatever responsibility there was, Italian and by Italians," Napolitano said.

Napolitano also expressed solidarity with the relatives of the victims and praised rescuers, including scuba divers, who "had made every effort" to rescue survivors or bring to shore the bodies of those who had died in the accident.

Italy's head of state made the remarks as he was exiting Rome's Basilica of St Mary of the Angels, where the memorial mass was celebrated.

During the solemn service, the head of Italy's top Catholic bishop and head of the country's episcopal conference, Cardinal Angelo Bagnasco, offered prayers for the victims of the accident.

The cruise ship's captain, Francesco Schettino, is under house arrest and faces charges of manslaughter, causing a shipwreck and abandoning the ship before all passengers and crew were evacuated.

Bagnasco also praised coast guard officials and rescuers as well as the inhabitants of Giglio, many of whom had opened their homes to accommodate survivors on the night of the accident.

"Italy regards them with esteem and gratitude. In them we recognise the deep soul of our nation, rich in intelligence and heart, always ready to do great things without losing its spirit," Bagnasco said.

But the cardinal also said "truth and justice" still needs to be done to "heal the wounds and restore trust."

The Concordia, carrying about 4200 passengers, was already listing heavily when an evacuation order was issued around an hour after the accident took place, officials say.

New video shows at times confused activity on the command bridge of the cruise ship as it began sinking.

At one stage, a voice from one of the people in the dimly lit command bridge is heard saying: "The passengers are entering the lifeboats by themselves."

To which someone - identified as captain Francesco Schettino by the TG5 news program that aired the footage - calmly replies: "Vabbuo (OK)."

The exchange apparently took place almost an hour after the ship ran aground and several minutes before an official order to abandon ship was issued.

In the aftermath of the accident, several survivors complained that they were not assisted by the crew during the evacuation and described scenes of chaos and panic.