A major fire in the Srisailam hydroelectric plant located on the Telangana-Andhra Pradesh border left nine people dead, most of them engineers, and three seriously injured, with thick smoke hampering rescue operations for over 12 hours,officials said on Friday.

A woman was among the dead.

Prime Minister Narendra Modi expressed grief over the incident.

President Ram Nath Kovind said he was pained by the loss of lives in the tragic incident.

Telangana Energy Minister G Jagadish Reddy said the fire broke out late on Thursday night when 17 people were inside the plant, run by the Telangana State Power Generation Corporation (GENCO).

Eight of the employees managed to come out, he said.

Thick smoke engulfing the area hampered immediate rescue operations, the minister said.

"A total of 17 employees were at the plant at the time of the accident. Eight of them were able to come out. But the remaining nine (seven GENCO employees, including a woman, and two private firm employees) who were trapped right there were tragically killed.

They tried to save the plant till their last breath," Reddy told reporters at the plant.

All the deceased appeared to have died due to asphyxiation though there were minor burns on their bodies, a senior official of GENCO told PTI.

The seven deceased belonging to GENCO included a deputy engineer and four assistant engineers. Two other victims were employed with a private firm engaged in battery maintenance in the plant.

Reddy said the employees who were trapped inside took videos of the fire through their mobile phones and posted the visuals to explain the situation to authorities outside.

A Fire Department official based on preliminary assessment said the blaze started in the battery area apparently due to "over voltage", which needs to be established after a thorough investigation.

Three employees who were seriously injured are being treated, according to a press release from GENCO.

Expressing grief over the tragedy, Prime Minister Narendra Modi tweeted "Fire at the Srisailam hydroelectric plant is deeply unfortunate."

"My thoughts are with the bereaved families.I hope those injured recover at the earliest," he said.

All the bodies were shifted to a local hospital for post-mortem.

Telangana Chief Minister K Chandrashekar Rao,expressing shock over the incident, ordered an inquiry by the CID.

Chief Secretary Somesh Kumar issued orders appointing additional DGP Govind Singh as inquiry officer, a press release from Rao's office said.

Nagarkurnool district Collector L Sharman said five fire engines were engaged in extinguishing the flames but thick smoke emanating from the plant's premises hampered rescue operations for over 12 hours.

The fire broke out at 10.30 PM on Thursday night and the rescuers could not proceed into the tunnel of the plant over the Krishna river until 12 PM on Friday.

Besides NDRF, CISF personnel also joined the state rescue teams.

The Srisailam Left Bank power station, about 200 km from Hyderabad, has six units with combined capacity of 900 MW (6X150).

Power generation was in full swing following incessant rains during the past few days, officials said, adding, power generation has since been suspended.

Though the fire was brought under control, thick smoke engulfed the underground area of the powerhouse making the rescue operations difficult, officials said.

The plant is located 125 metres under the ground in a long tunnel which has only one entry and exit point.

"The accident happened when a sudden fire broke out in the electric panels. Employees, who sensed danger made all efforts to extinguish the fire. The entrapped employees tried to save the plant from catching fire without even caring for their lives.

It appears that the employees continued their efforts until 12 o'clock at night (Thursday)," the power utility said in the statement.

One of the survivors, undergoing treatment at a hospital, told a TV news channel "We heard sounds..we saw flames and tried to put off the fire. But, we were unable to do so due to the smoke."