Suspected separatist rebels blew up a bus on the main highway in India's northeast Thursday, killing six passengers, police said.

Investigators said the attackers had planted a bomb on the Lilong Bridge in Manipur (search) state. The attack, which also injured 15 people, came a day before India celebrates its national independence.

No group immediately claimed responsibility for the attack, but the state's deputy inspector-general of police, D. Misra, said he suspected separatist rebels.

More than 30 rebel militias are operating in the northeastern states with demands ranging from secession to greater autonomy and the right to self-determination.

At least 11 separatist groups have called for a strike during the celebrations marking India's independence from Britain in 1947. The rebels asked citizens of the region to boycott all government functions and to refrain from any celebrations.

The rebels accuse New Delhi (searchof exploiting the natural resources of the northeast, which is rich in oil, minerals, tea and timber.

In another attack Wednesday evening in the region's Tripura (search) state, suspected rebels fired into a crowded market in the village of Jubotara, seven miles north of Agartala, the state capital, police said. Two people were killed and four were wounded.

The attacks are seen as part of the militants' offensive against security forces in the run-up to the Independence Day celebrations. In past years, the rebels have attacked vital installations, crude oil pipelines, trains, road and rail bridges, besides targeting federal troops fighting the insurgency.

Fearing attacks on trains, the Indian Railways on Thursday stopped night services in the region until Saturday. Troops were guarding government installations and conducting surveillance from helicopters.

The bridge explosion took place as the passenger bus carrying about 40 people crossed the Lilong Bridge, six miles south of Imphal, the capital of Manipur state, Misra said.

The victims of the attack were produce vendors and farmers.

"The bomb was planted on the bridge. We think they may have used a remote-controlled device," Inspector N. Sarat Singh told The Associated Press by telephone from the Imphal police control room.

The powerful blast ripped apart the front portion of the bus and left a four-foot-deep crater in the ground, Misra said.

Three passengers were killed immediately and three others died on the way to the hospital, police said. The injured were moved to a hospital in Imphal, Singh said.

Soldiers and police surrounded the area and were looking for the militants, who fled after the blast.