The death toll from an explosion at a luxury hotel in Havana's old quarter rose to 31 on Sunday, authorities said, as Cuban firefighters continued to comb through the rubble.

Twenty-four people were being treated in hospital, said the public health ministry, after the powerful blast Friday thought to have been caused by a gas leak. The previous death toll was 30.

Crews keep looking for victims and "until they are found I do not think there will be an end" to the rescue operation, said Alexis Acosta, the mayor of Havana's old quarter.

The five-star hotel had been closed for business during renovation work, and many victims of the late-morning explosion were construction workers or hotel employees preparing for its reopening, which was to have been next week.

The first four floors of the establishment were gutted in the late-morning blast that sent debris flying, smoke billowing into the air and rubble tumbling to the ground.

The explosion tore off large parts of the facade, blew out windows and destroyed cars parked outside the hotel, known for having hosted celebrities such as Madonna, Beyonce, Mick Jagger and Rihanna.

The dome of a nearby Baptist church also collapsed.

A 29-year-old Spanish tourist, who had been walking nearby, was among the fatalities, who also included four children and a pregnant woman.

The tourist's husband was injured in the blast that an official said happened while a gas tank was being refilled by a tanker truck.

Officials from the United States, Canada, the European Union, China, Russia and Venezuela have sent messages of condolence.