Militant attacks have sharply cut Nigeria's oil output in recent years
Royal Dutch Shell has shut three oil flow stations in Nigeria's Niger Delta region after a pipeline was sabotaged, a company spokeswoman has said.
She said Saturday's leak on the Trans Ramos oil pipeline was confirmed "to have been caused by sabotage". The leak was later stopped.
No group has so far claimed responsibility for the attack.
It came shortly after Nigeria's militant group Mend said it was ending the truce it declared last October.
Mend (Movement for the Emancipation of the Niger Delta) said it was not "directly responsible" for the attack, reports the Reuters news agency.
But an e-mailed statement said: "It was certainly a response to our order to resume hostilities by one of the various freelance groups we endorse."
Nigeria is one of the world's major oil producers but output has been cut by years of unrest in the Niger Delta - the source of its oil.
A Mend spokesman said the group did not believe the government would restore control of resources to local people.
Mend has demanded that local residents be given a greater share in profits from the oil produced by the region.
But much of the violence in the region has been carried out by criminal gangs.
Mend warned oil companies to prepare for an "all-out onslaught" against installations and personnel.
The BBC's Caroline Duffield in Lagos says it is not clear whether the statement speaks for the whole of Mend or just a faction that did not accept the offer of an amnesty from Nigerian President Umaru Yar'Adua.
Under the amnesty, thousands of fighters handed in their weapons last year.
Some militant leaders are not ready to return to violence, and have indicated they are willing to wait and see whether the amnesty process does move forward.
But it is also clear that for many others, patience is running out.
Correspondents say the peace accord has been jeopardised by Mr Yar'Adua's absence for the past two months - he is being treated for a heart problem in Saudi Arabia.