Hundreds of Maoists on Monday massacred at least 26 CRPF personnel in Chhattisgarh's Sukma district in the bloodiest attack in the state since 2013.

Prime Minister Narendra Modi described the deadly ambush on the 74th Battalion in a forested area in Kala Pathar near Chintagufa as "cowardly and deplorable" and said the deaths of the troopers won't go in vain.

The Central Reserve Police Force said the ambush began at 12.30 p.m., leading to a gun battle between the troopers and the Maoists who, survivors said, used hand grenades, automatic rifles and rocket launchers.

Troopers who survived the horror said the Maoists, women included, emerged out of the blue before opening indiscriminate fire.

CRPF Deputy Inspector General M. Dinakaran said 11 bodies were first recovered and a 12th trooper succumbed to his injuries in a hospital here. A subsequent search of the area led to 12 more bodies.

Two more men died subsequently in hospital, taking the toll to 26.

The 99-member CRPF patrol assisting a Road Opening Party was reportedly readying for lunch when it came under attack, taking the victims by surprise.

Air Force helicopters evacuated the wounded to hospitals in Raipur, officials said.

Security forces launched a major search operation to track down the Maoists.

This was the worst attack by Maoists in Chhattisgarh since 2010 when 76 CRPF troopers were killed. Twelve CRPF men were killed in a similar Maoist attack in Sukma early this year. 

Modi saluted the CRPF.

"We are proud of the valour of our CRPF personnel. The sacrifice of the martyrs will not go in vain," he tweeted. "Condolences to their families. May those injured in (the) attack recover at the earliest."

The Prime Minister said the government was monitoring the situation in Chhattisgarh closely. 

Home Minister Rajnath Singh said he was "extremely pained" by the killings and offered tributes to the dead and condolences to their families.

Congress President Sonia Gandhi and party Vice President Rahul Gandhi also condemned the killings of the CRPF personnel. 

"The sacrifice of 24 brave men is a big loss to the nation. Such attacks will never deter our fight against extremism," Sonia Gandhi tweeted.

Former Home Secretary R.K. Singh blamed the massacre on "complete intelligence failure on the ground level".

The killings forced Chhattisgarh Chief Minister Raman Singh to cut short his trip to Delhi where he had gone to attend a NITI Aayog meeting and fly back to Raipur. 

Maoists aligned to the outlawed Communist Party of India-Maoist have been active in Chhattisgarh's tribal-dominated interiors for decades, running a virtual parallel state.