Mend has carried out many attacks over several years
A militant group in Nigeria's oil-rich Niger Delta says it is ending the ceasefire it declared last October.
Jomo Gbomo, who said he was a spokesman for the group Mend, said it did not believe the government would restore control of resources to local people.
Mend has demanded that residents be given a greater share in profits from oil resources and land.
It warned oil companies to prepare for what it called an all-out onslaught against installations and personnel.
Analysts say it is not yet clear if this statement comes from the whole of Mend - the Movement for the Emancipation of the Niger Delta - or just a faction that did not accept the offer of an amnesty from President Umaru Yar Adua.
'Nothing will be spared'
In a statement announcing the end of the ceasefire, the group said that the decision had been taken "after careful consideration and extensive consultation".
"All companies related to the oil industry in the Niger Delta should prepare for an all-out onslaught," it went on.
"Nothing will be spared," it added, saying that the companies themselves would "bear the guilt" if their staff were harmed.
The BBC's Caroline Duffield in Lagos says the statement will raise fears of a fresh wave of kidnappings targeting foreigners.
Militants have carried out a series of attacks which have cost Nigeria millions in lost revenue over the years.
The attacks have meant that facilities in the area have been unable to work beyond two-thirds capacity, costing $1bn a month in lost revenue.
Despite the ceasefire, one Mend faction attacked a pipeline in December, saying it was a "warning strike" over what it called government delays in progressing with peace talks.
Talks have been held up by the long absence of President Yar'Adua in Saudi Arabia, where he has been undergoing medical treatment.
But our correspondent says he has made it is his personal project to end militant activity in the Delta and this development will increase the pressure on him to return.