Madrid, May 7 (IANS): Two people were reportedly missing after an explosion in the centre of Madrid, local emergency services said, adding that 18 persons also sustained injuries in the incident.

The explosion occurred at around 1.30 p.m. on Friday at number 35 Calle Ayala, in the exclusive Salamanca neighbourhood in the heart of the Spanish capital, reports Xinhua news agency.

Early indications are that the explosion was caused by a gas leak, although this is still under investigation.

On Friday evening, Madrid Mayor Jose Luis Almeida told mediapersons that "fatalities cannot be ruled out".

Meanwhile, the head of the Madrid Fire Department, Rafael Ferrandiz, explained that the missing people were two workers, who are thought to have been on the third floor of a building when the explosion occurred.

The workers could have fallen down to the ground floor patio, which had been covered by two metres of solid rubble, said Ferrandiz.

Firemen are working with dogs and drones to try to locate the workers.

The explosion has left the building in a "very unstable", condition, and there is a "a risk of collapse", Ferrandiz added.

Therefore, urgent work is being carried out to stabilise the building.

The three seriously injured people are an 84-year-old man who was taken directly to intensive care in a "very serious" condition, a 77-year-old woman with multiple head injuries, and a 44-year-old woman with leg injuries.