At least 13 people were killed in a fire in a Spanish nightclub on Sunday morning, authorities said, with fears the toll could still rise as rescue workers sift through the debris.

The fire broke out in the two-storey "Teatre" nightclub, also called "Fonda Milagros", in the city of Murcia in southeastern Spain in the early hours of the morning.

Emergency services said on X, the platform formerly known as Twitter, that firefighters were continuing to work at the scene and had not ruled out "the possibility of finding more victims."

Jose Ballesta, mayor of Murcia, said the fire had broken out at around 6:00 am local time (0400 GMT) and was "extremely serious."

Ballesta told press at the scene that "there are still bodies to be pulled out" from the rubble, which he said was a complicated task given the "risk of collapse."

Firefighters dispatched to the scene at 7:00 am were able to extinguish the fire by 8:00 am, he said.

Photos released by the emergency services show water hoses from fire trucks spraying the blackened facade of the club.

Thick smoke could be seen billowing from the roof of the building.

Authorities said four people were injured, two women aged 22 and 25 years old and two men in their forties, all suffering with smoke inhalation.

A police spokesperson said that a birthday party was being held at the club on Saturday night.

"According to initial information, the fire broke out on the first floor of the nightclub, which has a ground floor and a first floor," Diego Seral, the national police spokesman told radio Onda Regional de Murcia.

Forensic and judicial police experts "have been deployed to the Murcia nightclub where the fire broke out this morning to investigate the cause," the national police service said on X, formerly known as Twitter.

Video footage released by the city's firefighting brigade shows the firefighters holding a long hose approaching bright orange flames inside the venue, passing bar tables that still have drinks placed on top of them.

More than 40 firefighters and 12 emergency vehicles were attending the scene, authorities said.

Prime Minister Pedro Sanchez voiced "solidarity with the victims and relatives of the tragic fire in a Murcia nightclub."

The city's town hall had earlier said it "deeply regrets" the accident and offered condolences to those affected.

The mayor announced that three days of mourning would be held, and set up a reception base for the relatives of victims. (AFP)