Two vehicles carrying the troops on Tengnoupal-New Samtal road in Manipur's Chandel district went up in flames when hit by the RPGs.

17 soldiers died on the spot , three on way to hospital, officials said.

Most of the bodies were charred in the flaming vehicles.

But military spokesman Lt Colonel Ajay Kumar Sharma said the rest of the troops fought back and engaged the rebels in a fierce encounter for nearly three hours.

Location of ambush at Manipur's Chandel district. TIWN Pic

An insurgent was also killed but was carried away by the raiders.

Later reinforcements arrived from nearby military camps and the rebels fled.

This is one of the worst attack on the Indian army in the country's northeast, perhaps the most deadly in recent decades.

The Khaplang faction of the Nationalist Socialist Council of Nagaland (NSCN) and the United Liberation Front of Assam (ULFA) has claimed responsbility for the attack.

But Indian intelligence suspects a local Manipuri rebel group called KYKL ( Kanglei Yanna Kana Lup) was possibly involved in the attack.

Intelligence officials said about  15 KYKL and NSCN(K) insurgents, led by self-styled Lieutenant Tamba, were behind the attack. 

They doubt the ULFA's presence in the raiding party but said the Assamese rebel group is used to making claims in 'joint operations' even if they are just involved in non-combat tasks like publicity.

After the attack , the raiding group is said to have moved towards for Thantapin village south of Tamu in Myanmar where the KYKL is believed to have a few training camps. 

The NSNC(K), with which the Centre called off a ceasefire arrangement, operates from Myanmar’s Eastern Naga Hills in Sagaing Division.

The Manipuri rebel groups and several others from India's troubled Northeast have recently joined Burmese Naga rebel leader Khaplang in forming a -platform of rebel groups called the UNLFWSEA ( United National Liberation Front of West and SouthEast Asia).

Since then, the attacks on Indian forces in Northeast have sharply increased. One killed eight troops of the para military Assam Rifles in neighbouring state of Nagaland last month.     

Khaplang's faction of the once-united National Socialist Council of Nagaland (NSCN) reneged on a 14 year old ceasefire with India and kickstarted negotiations with other active rebel groups in the country's Northeast to form the ULFWSEA.

Khaplang is a Myanmarese Naga from the Hemi tribe.

Four groups have already joined them and some more might soon.

Indian intelligence says it suspects the involvement of the NSCN (Khaplang faction) with a seperatist group of Manipur in pulling off Thursday's ambush.

Khaplang however maintains a ceasefire with the Myanmar army he signed in 2012.

It has requested Myanmar to disarm and push out these rebels as India's other eastern neighbours Bhutan and Bangladesh has done in recent years.

But Myanmar says the terrain in Sagaing is treacherous and its troops are busy fighting other rebel groups like the Kachin Independence Army (KIA) and the Kokang rebel group MNDAA on the borders with China.

The spurt in rebel activities follows the breakdown of ceasefire between Delhi and the NSCN's Khaplang faction.

India's Northeast has been home of dozens of insurgent groups, some fighting for independence, others for greater autonomy and still some for separate states within India.