KORAPUT/ BHUBANESWAR: The Maoists, who were lying low for the past several months in Odisha, triggered a landmine blast on Tuesday blowing up four BSF men to pieces on a national highway near Koraput in an apparent revenge attack for the police killing their leader Madhav in an encounter in the district four days ago.

Such was the intensity of the blast that it tossed security vehicles several meters away from the site apart from creating a five feet deep crater on the highway that blocked traffic for the entire day. Those killed included an assistant sub-inspector, while three more security personnel were injured.

The attack took place around 9am between Kauguntha and Sakirai village on NH-26 along Odisha- Andhra Pradesh, about 50 km from Koraput district headquarters town, in Pottangi area. Chief minister Naveen Patnaik said a convoy of three BSF vehicles carrying 18 men was moving from Malkangiri to Visakhapatnam on "administrative duty" when the rebels struck, blowing up one of the vehicles.

"The remaining BSF personnel fought back. Reinforcements have reached the spot," assured Patnaik.

Security personnel of BSF's 161 Battalion were deployed at Balimela in Malkangiri district, notorious for insurgent activity. "They were on their way to the headquarters via Visakhapatnam," a source said.

"The rebels first fired at the second vehicle from a small hillock on the left of the hill when the jawans stopped the vehicle. That's when the Maoists triggered the landmine, exploding the third vehicle. Each vehicle carried six jawans," a police source said.

Those killed were ASI Jaswant Singh (Punjab), driver Surender Singh (Himachal Pradesh), havildars Virendra Prasad and Gurdayal Singh (both from UP). The injured Mahender Kochker and Siraj Julla were shifted to a hospital in Visakhapatnam, while inspector Gaurav, who was hit by a bullet in his stomach during the gun-battle, was admitted to Koraput hospital.

"It's a setback but we'll definitely strike back," said DGP Prakash Mishra. He said combing operations had been intensified after the attack.

Soon after the blast, the Maoists, numbering around 40 to 50 and belonging to the Andhra-Odisha Border Special Zonal Committee, began firing from a hillock at the jawans. The security men retaliated.

"The firing took place for quite some time, but the Maoists escaped. Additional forces, which reached the spot, recovered a hand grenade, a landmine, wire and food stuff from the hill where the rebels were hiding," said SP (Koraput) Awinash Kumar, who narrowly escaped an ultra bullet.

In previous attacks in the region, four BSF officers were blown up in a landmine attack at Balimela in Malkangiri in 2012 and nine SOG jawans killed in a landmine blast at Boipariguda in Koraput in 2010.