Police in India's north-eastern state of Assam have arrested three men and are investigating whether separatist militants received support from terrorist groups to carry out multiple bombings that claimed 81 lives, news reports said Saturday.

Among those arrested overnight were a man whose car was used in one of Thursday's bombings in the state's capital city of Guwahati, the IANS news agency reported quoting the state's senior police official Bhaskar Jyoti Mahanta.

Another arrested man's motorcycle was used in a bombing in the western Bongaigaon district.

The third arrest was made in eastern Morigaon district. Police said a man from whose mobile phone a text message was sent in the name of a little-known terrorist group claiming responsibility for the bombings was being questioned by the police.

The SMS was sent to a local television station News Live on Friday in the name of "The Islamic Security Force (Indian Mujahideen)" warning of more such attacks.

Police said the group came into existence in 2000 to counter attacks by Bodo tribals on Muslim settlers from Bangladesh in Assam.

Police and security agencies were investigating details of the group and said the name resembled that of Indian Mujahideen, which claimed responsibility for bombings in Jaipur, Ahmedabad and Delhi this year that killed over 140 people.

But senior state security officials said the state's major separatist group, the United Liberation Front of Asom (ULFA), was still the key suspect in the bombings.

India's Home Secretary Madhukar Gupta earlier told reporters that the blasts could be the work of local rebel groups having "external linkages" - an apparent reference to the Bangladesh-based Islamist group Harkat-ul Jehadi Islami (HuJI).

In an email sent to news organizations shortly after the blasts, the ULFA, denied any involvement in the attacks.

The ULFA, Assam's largest militant group, has been fighting Indian security forces for an independent homeland for the Assamese over the last three decades.

The ULFA blames the Indian government for neglecting the development of Assam while exploiting its resources, including oil.

Indian security officials in the past have alleged that some key members of the ULFA are hiding in Bangladesh and have forged ties with HuJI and other terrorist groups.

Assam was rocked Thursday by a wave of bombings - 12 blasts in quick succession - six in Guwahati, and six in the three western districts of Barpeta, Kokrajhar and Bongaigaon.

The death toll in the bombings rose to 81 on Saturday with four more people succumbing to their injuries at hospitals in Guwahati, police said.

More than 400 people were injured as the bombs placed in cars, autorickshaws and drains in busy market areas exploded within minutes of each other in the worst terrorist strike the state has seen. Forty- five people died in Guwahati alone.

Meanwhile, Prime Minister Manmohan Singh who is an elected member of parliament from Assam flew to Guwahati on Saturday.

Singh rejected the opposition charge that his government was soft on terror.

"We are not going soft on terror. There can be no compromise with terror," Singh told reporters after visiting the blast victims at hospitals.

"Whosoever is responsible for these dastardly acts... we will take effective measures," he added.

Local media reported that people in the state capital burnt effigies of Singh to protest against the government's inability to curb terrorist strikes and protect lives and property.

Singh was later scheduled to meet with top state officials to have a first-hand assessment of the security situation in the state.