(Reuters) - India has sought cooperation from Bhutan, Myanmar and Bangladesh in an offensive against a tribal militant group that had shot dead at least 80 people in Assam this week, officials said on Friday.

Police believe that a faction of the National Democratic Front of Bodoland (NDFB), fighting for a separate state for ethnic Bodos, was behind coordinated attacks on tea plantation workers and their families this week, the deadliest in years.

Some militants may have fled to neighboring Bhutan while their leader was believed to be in Myanmar, officials and police said, prompting calls for cooperation.

"We are determined to hunt down the rebels and destroy their base, both in the country and outside," Minister of State for Home Kiren Rijiju told Reuters.

Assam is one of the seven states that make up India's remote northeast, home to more than 200 tribes and dozens of insurgencies, some seeking greater autonomy and others secession.

The region has trailed the rest of India in economic development and the gap has widened in recent years, fuelling discontent. Residents accuse the central government of plundering the natural resources while ignoring development.

The latest attacks, in which half the victims were women and children, have shaken Prime Minister Narendra Modi's government that came to power promising economic growth as well as a militarily secure India.

"This is terrorism, there is zero-tolerance for terrorism," Home Minister Rajnath Singh, who flew to Assam to commiserate with the families of the victims, told reporters.

The northeast is wedged between China, Myanmar, Bangladesh and Bhutan, and militants are known to crisscross borders that run through thickly forested mountains.

Assam police say the leader of the NDFB, I.K. Sonbijit, is based in Myanmar, from where he controls his cadres.

He is believed to have ordered this week's attacks in retaliation for a army offensive against his group in which he lost 40 men and a large quantity of arms and ammunition.

Some 7,000 villagers have fled their homes in Assam, fearing more attacks from the Bodo militants and are sheltered in relief camps guarded by the army and police.