A bomb strapped to a bicycle exploded in a market in Assam's district of Sunitpur on Sunday, wounding at least 15 people. A government officer blamed the attack on Bodo tribesmen who are fighting for a separate homeland.
Three men, said by police to be members of the underground National Democratic Front of Bodoland (NDFB), died when another bomb they were carrying went off in another part of Sunitpur.
The explosions happened hours before Indian Home Minister Shivraj Patil arrived for a security review of Assam and Nagaland in the strategic northeast region bordering China, Myanmar, Bangladesh and Bhutan.
"The NDFB, which was lying low for quite some time is now desperate," Assam Home Commissioner Biren Kumar Guhain said. "They have carried out these attacks to boost the morale of their cadres."
Sunday's explosions followed a wave of bombings and attacks in Assam and neighbouring Nagaland on Saturday that killed dozens of people and sparked concern that a seven-year-old peace process with separatist Naga rebels could be disrupted.
Troops had begun search operations for hideouts of the United Liberation Front of Assam and the NDFB in the thick jungles of western Assam, a police officer said.
The Assam border with Bangladesh
has seen increased tension recently
In the neighbouring state of Nagaland, troops armed with machine guns patrolled the streets, as mourning families buried the victims of a wave of bombings and gunfire that killed 46 people in the northeast region.
Three bombs tore through a marketplace and a railway station on Saturday in Dimapur, Nagaland's commercial hub, killing 26 people and wounding more than 50 in the deadliest attacks since a ceasefire with the main Naga rebel group began seven years ago.
"There is silence in the town. Troops are on the streets with machine guns," said Dimapur resident Ato Jamir. Families were burying the dead in accordance with tribal custom, he
UN Secretary-General Kofi Annan condemned the attacks in Nagaland and Assam, saying no grievance could justify the killing of innocents.
"The secretary-general has learned with shock and dismay the news of bomb and gunfire attacks that took place today in crowded public places in Nagaland and Assam states, India, which have claimed the lives of a large number of people," a statement said.
UN chief Annan has condemned
It was the worst attack in Christian-majority Nagaland since the main separatist group National Socialist Council of Nagaland (Isaac-Muivah) signed a truce with the Indian government in 1997 and began talks to end a long-running insurgency.
The NSCN (I-M) said underground groups opposed to peace in Nagaland could have been behind the attacks and vowed to hunt them down.
"We will find out whoever has done it. This is the handiwork of forces opposed to the ongoing peace process," said Phungthing Shimray, a senior NSCN leader.
The group announced a reward of 100,000 rupees ($2170) for information on the attacks.
India's mountainous northeast is home to dozens of underground groups, some fighting for greater autonomy, or statehood and others for secession.
The groups accuse the federal government of plundering the region's rich resources and neglecting the local economy.
Security analysts say successive Indian governments have largely ignored the northeast, focusing almost entirely on the rebellion in the disputed northern region of Kashmir.