Heavily armed rebels in India's restive northeast killed at least 20 troops on Thursday, police said, in one of the area's worst such attacks in years.

Another 12 soldiers were seriously wounded in the ambush on a military convoy in Manipur state, which has been plagued by insurgent violence for decades.

"Preliminary reports suggest that militants used rocket-propelled grenades on a convoy, killing 20 army soldiers on the spot," Manipur's home secretary J S Babu told AFP by telephone.

Police said the convoy was in a remote area of Chandel district, about 120 kilometres (75 miles) southwest of Manipur's state capital Imphal, when the attackers struck.

Police said details remained scant because of poor communications in the remote area, which is deep inside a forest.

The wounded have been taken to a military hospital in Manipur.

No militant groups have so far claimed responsibility for the ambush, which police and the military said was one of the biggest attacks on security forces in the state in recent years.

"Some of the vehicles in the convoy were blown up beyond recognition," said a senior army official on condition of anonymity.

"You can well imagine the intensity of the attack."

The remote state of Manipur, which borders Myanmar, is home to dozens of tribal groups and small guerrilla armies fighting New Delhi's rule. Their demands range from secession to greater autonomy.

Many in the northeast feel the region has been largely neglected by India's political leaders, accusing them of focusing on the development of the country's wealthier states.

"We cannot say who is behind the attack," said S. Manglemjao Singh, inspector general of police for the state.

"We are collecting details about the attack, but preliminary reports suggest a large group of militants who were heavily armed triggered an IED (Improvised Explosive Device) blast and later opened fire with assault rifles."

At least 50,000 people have lost their lives in insurgency-driven violence in six of India's seven northeastern states since the country's independence from Britain in 1947, authorities say.

The seven states are home to dozens of guerilla and tribal groups, which often compete against each other.

The army said it had launched a search for the attackers.

"We don't know how many attackers were part of the ambush," said spokesman Rohan Anand.

"The operation is still on and we have launched a search operation in the area to neutralise the attackers."

Defence Minister Manohar Parrikar condemned the attack in a tweet and pledged to bring those responsible to justice.