Rescue workers have pulled another four bodies from the ruins of a Havana luxury hotel wrecked in an explosion last week, raising the death toll to 35, an official said.

Crews continued to comb through the rubble of the Saratoga five-star hotel, which was being renovated and had no guests at the time of the late-morning blast last Friday that was seemingly caused by a gas leak.

The search is now focused on the basement and sub-basement of the building.

"It is at a very dangerous stage because of the concentration of rubble and the danger of collapse," fire department head Luis Carlos Guzman told state television Monday, updating the death toll.

At the time of the blast, said the tourism ministry, there were 51 workers inside the hotel, which was readying for its post-refurbishment reopening this week.

The ACN state news agency said the four bodies found overnight were those of hotel workers.

"According to family members, it is estimated that there are about 12 or 13 people still trapped," the news agency added.

On Sunday, officials said 24 people were receiving treatment in hospital.

The first four floors of the hotel were gutted in the 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.