At least 22 members of Indian security forces were killed in a central Indian state by Maoist fighters, in one of the bloodiest attacks by the extreme left-wing insurgent group this year, officials said on Sunday.

Security personnel belonging to the Central Reserve Police Force’s elite CoBRA unit, the District Reserve Guard, and the Special Task Force were attacked on Saturday in the tribal-dominated Chhattisgarh state during an anti-insurgency operation.

“We can confirm that 22 of Indian force members have been killed by Maoist fighters,” said a senior government official in Raipur, the capital of mineral-rich Chhattisgarh.

They were killed in firing that lasted for four hours in the border district of Sukma, 540 kms (340 miles) south of Raipur.

Om Prakash Pal, a senior police official in Raipur, said combing operations to trace one missing security force member were being conducted.

The death toll was the heaviest for Indian security forces battling the far-left guerrillas since 2017.

Home Minister Amit Shah said the government will not tolerate such bloodshed and “a befitting response will be given to prevent such attacks.”

The Maoists, also known as Naxals, have waged an armed insurgency against the government for decades. Leaders of the hardline leftist militant group say they are fighting on behalf of the poorest, who have not benefited from a long economic boom in Asia’s third-largest economy.

Since 1967 the group seen as the greatest threat to the country’s internal security have asserted control over vast swathes of land in central and eastern India, establishing a so-called “red corridor”.

The militant group operate from thick forests, and their operations against the Indian administration and forces are shrouded in secrecy. The group has not claimed responsibility for the latest attack.

Hours after the attack, Prime Minister Narendra Modi wrote on Twitter that the “sacrifices of the brave martyrs will never be forgotten. May the injured recover at the earliest.”

Last month, five policemen were killed and several others were injured in a blast in what police said was a Maoist strike targeting a bus carrying more than 20 security personnel.