Additional forces have been sent to Assam

They say that around 20 people were injured, two of them seriously.

The authorities have accused the separatist United Liberation Front of Assam (Ulfa) of carrying out the attack and another attack on Wednesday.

Ulfa is fighting one of India's longest running insurgencies to establish an independent homeland in Assam.

Ceasefire

Meanwhile, fresh trouble has erupted in the state's western region inhabited by Bodo tribes people after police arrested a rebel National Democratic Front of Bodoland (NDFB) separatist leader, Gopal Rabha.

He was arrested in Guwahati this week in connection with a separate bomb explosion in a train in West Bengal state in November.

The NDFB is now observing a ceasefire with the government.

But police say that on Thursday NFDB supporters angered over Mr Rabha's arrest set fire to a West Bengal transport bus and two trucks in the west of Assam.

They also closed down the highway that connects Assam with the rest of the country, severely disrupting traffic.

Last week thousands of soldiers were ordered to fan out across the state in search of Ulfa rebels who police said had killed at least 70 people. Many of the victims were Hindi-speaking migrants.

The rebels say India's central government exploits the state's rich resources such as tea and natural gas and does little for its people who are ethnically closer to Burma and China than to India.