In a separate incident, also in Assam, seven people were wounded when a hand grenade was thrown at a police vehicle.

In the ambush, the militants opened fire on two vehicles carrying railway workers and the soldiers guarding them, said Assam state police chief DN Dutt.

"The incident took place at 10:30 this morning," said Dutt. "We suspect it to be the handiwork of a tribal militant group in the area called Dima Halam Dauga," he said.

The group represents the Dimasa, a small ethnic group fighting for more autonomy. The militants operate in the area of the attack, some 360km south of Gauhati, the capital of Assam.

Four other railway workers were wounded in the ambush and the rebels fled with the soldiers' guns, Dutt said.

In the other incident, separatist rebels in the town of Doomdooma hurled grenades at a police vehicle, injuring three policemen and four bystanders, authorities said.

"A grenade was lobbed at a senior police officer's vehicle Thursday night ... but it missed the target and exploded seconds later," said local government official Absar Hazarika.

Hazarika blamed the attack on the United Liberation Front of Asom, or Ulfa, which has been campaigning for Assam state's independence from India for the past 26 years.

Doomdooma, about 550km east of Gauhati, Assam's capital, is a tea-growing area and a Ulfa stronghold.

The federal government unilaterally announced the truce with Ulfa in August as a goodwill gesture, but canceled it after rebels attacked a police patrol and killed a tea planter in late September.

At least 10,000 people, mostly civilians, have died in Assam since the insurgency began more than two decades ago.

There are also several smaller militant groups fighting for the rights of ethnic minorities operate in the area.

The militants say India's central government exploits the northeast's rich natural resources while doing little to improve its poor infrastructure or to alleviate widespread unemployment.