The van carrying the members of the Special Operations Group was on its way from MPV 126, 50km from Malkangiri, to MPV-41, when the landmine went off.
The SOG personnel were going to the place where the house of a BJP leader was attacked by the Maoists on Tuesday night.
''We suspect that all the 21 personnel in the van were killed in the incident,'' Director General of Police Gopal Chandra Nanda said.
The SOG personnel were ambushed by the Maoists who felled trees to block Kalimela-Motu road to block the vehicle. When some of them alighted to clear the road they were fired on by the Maoists and their vehicle was damaged by the landmine explosion triggered at about the same time, police said.
Though the vehicle was not completely damaged in the blast as it was an anti-landmine one it had overturned and those inside it were gunned down by the ultras when they tried to come out from it.
Security forces were rushed to the spot after the explosion that came 18 days after the Maoists carried out an audacious attack on a launch carrying elite anti-Naxal Greyhound police personnel of Andhra Pradesh in a reservoir in Malkangiri district. That incident left 35 dead and dozens injured.
The south western range DIG Sanjeev Panda said the van was an anti-landmine one and there were 21 personnel in it.
He, however, did not confirm the casualty figure. "Our people are yet to reach at the explosion site".
Another anti-landmine vehicle was sent to the spot to rescue the personnel, the police said.
Wednesday's Naxal attack came on a day when senior officials of the Central and governments of four severely Naxal-affected states, including Orissa, confered in New Delhi to address the increasing security threat from Maoists.
Earlier this month, the Naxals had opened fire on a launch carrying Greyhound commandos who were on a patrol in Orissa, sinking the vessel. While 29 policemen survived the attack with serious injuries, bodies of 35 others were recovered during a search that lasted several days of search.
It was one of the biggest Maoist assaults on security personnel since March, 2007 when the ultra-Left rebels blew up a vehicle killing 55 poliemen in Chhattisgarh.