Nine persons are feared trapped in Srisailam hydroelectric plant located underground on the Telangana-Andhra Pradesh border after a fire broke out on Thursday night and "huge smoke" was preventing rescue personnel from entering in, officials said on Friday.

Nagarkurnool district Collector L Sharman said three fire engines were engaged in extinguishing the huge smoke emanating from the plant's premises.

While Chief Minister K Chandrasekhar Rao was monitoring the rescue work through his cabinet colleague, Union Minister of State for Home, Kishan Reddy, said he has asked NDRF to join the rescue efforts.

"There is huge smoke and because of it we are not able to enter into the plant premises. By 11 am we expect to enter the plant," Sharman told P T I, adding, nine persons including seven staff members are stuck.

Those stuck include a Deputy Engineer and an Assistant Engineer.

"Our main target is to rescue them...we are not able to contact them because they are not carrying mobile phones and there is no mobile network," he said.

Quoting preliminary information, he said a short- circuit is suspected to have caused the fire.

According to B Suresh, Chief Engineer of Telangana State Power Generation Corporation (GENCO), there were nearly 25 people at the powerhouse when the mishap took place.

However, about 15 or 16 managed to come out.

A police official said six persons have been admitted to local hospitals for treatment for suffocation.

Built on river Krishna, the Srisailam Left Bank power station, about 200 km from Hyderabad, is operated by the Telangana State Power Generation Corporation (GENCO).

The plant has six units with combined capacity of 900 MW (6X150). The power generation was in full swing following the incessant rains during the past few days.

Power generation has been suspended following the fire mishap.

The fire has been brought under control but thick smoke engulfed the underground area of the powerhouse making rescue operations difficult, officials said.

Two of the nine who are trapped belong to a private firm that maintains batteries, officials said.

"There are three emergency exits in the pant. There is a possibility that they (trapped) can come out through them.

But we cannot say until the smoke is cleared and the rescue teams can go inside," Suresh said.

The plant is located in a one km long tunnel and the fire is suspected to have broke out at the service bay area in the Main Control Room, a Fire Department official said.

"We were not able to reach the site as dense smoke engulfed the area. As of now we are trying to put off small patches of fire here and there at the transformer area," the official said.

Telangana Energy minister Jagadish Reddy said personnel from Police, Fire and Emergency services department are involved in the rescue operations.

Officials of the Singareni collieries have also been contacted for their help since they have expertise in underground rescue operations, he added.

Chief Minister Rao expressed shock over the accident.

"The CM is eliciting information on the accident on a regular basis. The CM wants all those who are trapped in the plant to come out safely," a press release from his office said.

Rao spoke to Minister Jagdish Reddy and Transco, Genco CMD D Prabhakar Rao who are at the site and reviewed the relief measures taking place there, it said.

Kishan Reddy said he spoke to the NDRF personnel and directed them to immediately get involved in rescue operations.

"Saddened by the fire accident in the Hydel Power Generation Unit on the left banks of Srisailam, Telangana, late night. Spoke to the NDRF personnel and directed them for immediate rescue operations. Prayers for early recovery of the injured" he said in a statement.

Telangana BJP in a statement demanded the government to put to use all resources to provide medical support to the injured immediately, and ensure the rescue and relief are undertaken on war footing to find those who are still stuck inside the plant.