An explosion was reported at a government building hosting amnesty talks in the southern Nigeria oil town of Warri Monday, after militants issued a bomb threat.

The Movement for the Emancipation of the Niger Delta (MEND) said in an e-mailed statement that its operatives had planted three bombs in the compound of the building and would detonate the first device at 11.30 am (1030 GMT).

The group gave only 30 minutes notice of their intention to detonate the device and allow evacuation.

Reports said that the device went off near a hall where a conference to discussing last year's government amnesty for oil militants was underway.

It was not immediately clear if there were any casualties.

MEND spokesman Jomo Gbomo said that the group would release details of when the other two bombs would be detonated and warned this was the beginning of a new onslaught.

'In the coming days, we will carry out a number of attacks against installations and oil companies across the Niger Delta,' he said.

The attack is the first MEND action in almost five months, after the group called a ceasefire in response to the amnesty.

MEND says it is fighting for a share of oil revenue for Niger Delta residents, who complain that multinational oil companies have ruined their agriculture and fishing livelihoods and caused environmental damage in the deltas creeks.