At least four paramilitary troopers were killed and about 30 injured in a powerful landmine explosion in Assam Friday, police said. The outlawed United Liberation Front of Asom (ULFA) claimed responsibility for the explosion in the restive northeastern state.

A police spokesperson said militants triggered the remote-controlled landmine when Central Reserve Police Force (CRPF) troopers were returning from a training session near village Holmari in Goalpara district, about 130 km west of Assam's main city of Guwahati.

'Forty CRPF troopers were returning back to their base camp after a routine anti-insurgency training exercise when the landmine explosion took place,' a senior police official said.

Four CRPF troopers were killed on the spot and about 30 injured. The injured were shifted to local hospitals in Goalpara town with multiple injuries, the official said.

'The condition of at least five of the injured is stated to be critical,' he added.

ULFA claimed responsibility for the explosion.

'We carried out the blast on the CRPF bus and will step up our offensive if the government does not immediately stop killing our cadres,' ULFA spokesperson Anu Buragohain told a local TV channel by phone.

'We have noticed the government's attitude, thinking we are a spent force. But then they are wrong in thinking like that,' he added.

Earlier, the police claimed that the blast was carried out by suspected militants of the outlawed National Democratic Front of Bodoland (NDFB).

'We don't think ULFA has any base here, although we know for sure that the NDFB has a strong presence in the district,' the police official said.

A massive security operation was launched in the area with police and CRPF troopers, along with army soldiers, combing the location to hunt for the rebels.

'The area where the explosion took place is a thickly forested one and the militants must have taken advantage of the location,' the official said.

This is the second biggest attack on security forces in Assam this week - four troopers of the Sashastra Seema Bal (SSB) were killed and seven injured in an ambush Monday by NDFB militants in the western district of Chirang.