Video footage broadcast on local television showed huge flames leaping through barred windows of a one-storey wooden hospital ward.
It took some three hours for 10 squads of firefighters and rescuers from the emergencies ministry to extinguish the blaze, the APA news agency reported.
President Ilham Aliyev, who went to the scene, has set up a governmental commission to probe the cause of the blaze and ordered financial aid to be allocated for the families of the victims, his office said.
“At 06:10 am a fire broke out at the Republican Narcological Centre in Baku,” the General prosecutor’s Office, the health, interior and emergencies ministries said in a joint statement.
“According to the latest data, 25 people died in the fire,” said the updated statement, which initially put the death toll at 24.
Four more people were hospitalised with serious burns, the statement said, citing a power grid defect as the initial cause of the blaze.
More than 200 patients and medical staff were evacuated unharmed, the statement added.
– ‘Systemic corruption’ –
The General Prosecutor’s office said it has launched a criminal investigation over alleged violation of fire safety norms.
Russian President Vladimir Putin sent condolences to his Azerbaijani counterpart, according to the Kremlin’s website.
Civil activists blamed official corruption and the failure to respect safety regulations as the factor causing large numbers of casualties in increasingly frequent fires in Azerbaijan.
“Fire safety regulations were not respected in the hospital’s Soviet-era wooden building,” the head of the “Property Rights” civil advocacy group, Aitekin Imranova, told AFP.
“As a result of systemic corruption, an adequate control over the compliance with fire safety norms is not in place in Azerbaijan,” she said.
Earlier Friday, APA reported that at least 30 people had perished in the blaze which it said broke out in a ward for bed-ridden patients.
The energy-rich, ex-Soviet Caucasus nation has a history of large-scale casualties as a result of fires in residential buildings and elsewhere.
In May 2015, 15 people – including five minors — died in a fire in a multi-storey building in Baku.
In October 1995, 289 people died in a metro fire in the capital, in the world’s deadliest subway disaster that was caused by outdated Soviet equipment.
Rights groups have long accused the government of authoritarian President Aliyev of widespread corruption that undermines functioning of state institutions.