Guwahati: Thousands of soldiers backed by helicopters launched a major offensive against the ULFA's bases in jungles of Assam and Arunachal Pradesh in the wake of the killing of 70 migrant workers by the banned group.

As helicopters hovered overhead, soldiers along with paramilitary and police forces fanned out in forests where the ULFA is believed to have set up new bases, especially in upper Assam and bordering areas of Arunachal Pradesh.

There were reports of three militants being gunned down and an equal number being arrested from a reserve forest in Darrang district.

"A major offensive has been launched in Sibsagar, Tinsukia and Dibrugarh and districts in upper Assam which will be backed up by airborne troops," said Maj Gen N C Marwah of 2 Mountain Division.

"With the arrival of additional forces from the Centre and lower Assam, the army has fanned out in the three districts to conduct concentrated operations", Marwah said.

Security was tightened along Assam's borders with Tripura and Meghalaya and an alert sounded so that militants could not sneak into the neighbouring states.

No fresh violence against migrant workers in Assam has been reported since yesterday, officials said.

Chief Minister Tarun Gogoi reviewed the offensive against the ULFA at a meeting of the Unified Command at Dispur here. He said an additional 20 companies of army and paramilitary forces (around 2,000 personnel) had arrived in the state.

Officials said the operations in Arunachal Pradesh were aimed at flushing out the ULFA which has bases in the state to carry out hit and run guerrilla strikes or to escape to the Myanmar border.