Iranian police on Wednesday questioned four people as part of investigations into a powerful explosion that killed 19 people at a Tehran clinic the night before, state media reported.

The blast at the Sina At'har health centre in the upmarket northern neighbourhood of Tajrish caused damage to nearby buildings and sent a plume of thick black smoke into the sky. Fifteen women were among the 19 people who lost their lives in the tragedy.

It was the second such incident to hit Tehran within days, after a gas tank explosion near a military complex east of the capital late last Thursday that authorities said caused no casualties. "The police are questioning four people and investigating them," Tehran's deputy police chief General Hamid Hadavand was quoted as saying by Iribnews, the website of state television.

Among them were the director general and three other officials from the clinic, which was badly damaged by the blast and a fire that took two hours to extinguish.

Tehran's police chief General Hossein Rahimi on Wednesday denied the incident could have been the result of "sabotage". "These are definitely rumours," he was quoted as saying by Fars news agency.

The tragedy was the result of a "fire (that broke out) in this private health centre," he added. The cause of the incident is still unknown, however. Iran's President Hassan Rouhani, in a message of condolence to the families of the victims, called on the authorities concerned to shed light "on the cause of the accident".

Tehran fire brigade spokesman Jalal Maleki said the explosion occurred when a fire spread to gas canisters stored in the basement of the clinic.  Deputy Health Minister Iraj Harirchi denied rumours circulating on social media about "the presence of radioactive materials" in the building. "The Sina At'har clinic was not a nuclear medicine centre" but "a dental and radiology centre," he said.