By Subir Bhaumik BBC News, Calcutta

The United Liberation Front of Assam (Ulfa) and three other rebel groups in north-east India have urged supporters to boycott Republic Day this month.

A joint statement condemned India's "illegal occupation" and called for a general strike on 26 January.

An Ulfa spokesperson also confirmed the group had told India's state-run oil company ONGC to pay 5bn rupees ($113m) if it wanted to operate in Assam.

Peace moves between Delhi and Ulfa look as good as over after three months.

On Friday, Ulfa fighters lobbed hand grenades near an oil refinery at Noonmati not far from Assam's capital, Guwahati.

Pipeline explosion

The grenade attack, which injured three people, comes after Ulfa separatists bombed an oil pipeline earlier in the week on the eve of Prime Minister Manmohan Singh's visit to the state.

The joint statement issued by Ulfa - along with the All Tripura Tiger Force, the Manipur People's Liberation Front and the Kamatapur Liberation Organisation which is fighting for a tribal state in northern Bengal - says they have called for a general strike on 26 January.

"We urge our people to stay away from Indian Republic Day ceremonies because these occasions only serve to consolidate Indian illegal occupation of our region," the joint statement said.

While separatist groups in north-east India oppose Republic Day and Independence Day celebrations as a matter of routine, Ulfa was not expected to take such action this year.

It had taken some steps to initiate a dialogue with Delhi.

Three months ago, it set up a committee of civil society leaders in Assam to carry forward the peace process with the Indian government.

The prime minister, along with senior cabinet colleagues, met the so-called People's Consultative Group in Delhi to express his commitment to peace in Assam.

But there has been no further meeting of the committee and the Indian government.