Nigeria's main militant group said its fighters had attacked an oil facility belonging to Royal Dutch Shell in the Niger Delta on Monday, days after President Umaru Yar'Adua proposed an amnesty.

Shell said it had shut in some production as a precautionary measure while it investigated reports of attacks on two well clusters in its Estuary Field in the western Niger Delta, which feeds into its Forcados oil export terminal.

A senior industry source said the latest attack, following similar strikes against U.S. energy firm Chevron and Italy's Agip over the past month, meant virtually all oil output in the western half of the delta was shut in.

"Basically in the western Niger Delta there is no production," the source said, asking not to be named.

The western Niger Delta accounts for roughly half of output from the world's eighth biggest oil exporter, which has an installed capacity of around 3 million barrels per day (bpd).

The OPEC member was currently producing around 1.74 million bpd, state oil firm NNPC said, buoyed by higher output from its Bonny Light stream in the eastern delta.

Oil rose near $72 a barrel on the attack.

"The Nigerian situation is the main factor in the market," said Mike Wittner, global head of oil research at Societe Generale. "The attacks appear to be removing some oil production capacity from the market."

The Movement for the Emancipation of the Niger Delta (MEND) said it had struck at Shell's Forcados operations at about 3:30 a.m. and that parts of the facility were on fire.

Forcados is one of Nigeria's benchmark crude oil grades but output has been interrupted by militant attacks before. Shell already had a force majeure on its Forcados shipments for the rest of June and July, freeing it from contract obligations.

Chevron said last month it had shut 100,000 bpd from the western delta while Agip declared force majeure last week on Brass River exports, also in the western delta.

Yar'Adua on Thursday offered a 60-day amnesty to gunmen to try to end unrest which has prevented Nigeria from pumping much above two-thirds of its installed capacity, costing it billions of dollars a year in lost revenue.

AMNESTY DIVIDES MILITANTS

Some militant leaders have said they want talks with Yar'Adua to work out the details of a deal, but MEND has publicly dismissed the amnesty offer, seeing it instead as an opportunity to distinguish itself from criminals.

"It will separate the wheat from the chaff and allow the government to focus on the root issues instead of tying militancy with criminality as an excuse for not addressing the grievances of the Niger Delta people," it said.

"MEND will negotiate as a group when the right time comes ... Only those who are willing to sell their birthright for a bowl of porridge will accept while the rest of us will continue the struggle until justice is achieved."

Representatives of Ateke Tom, Farah Dagogo, Soboma George and Boyloaf -- key leaders of armed gangs behind some of the most spectacular attacks -- issued a statement on Friday saying they wanted to meet Yar'Adua.

MEND -- a loose coalition of various armed gangs in the delta -- denied Dagogo and Boyloaf would take part.

One of MEND's key demands has been the release of its suspected leader, Henry Okah, who is on gun-running and treason charges and could face the death penalty if convicted.

A presidential spokesman said Okah, who was arrested in Angola in September 2007 and extradited to Nigeria five months later, would be freed if he took the amnesty offer.

MEND says it is fighting for a fairer share of the oil wealth in the Niger Delta, one of the world's largest wetlands where villages remain mired in poverty despite five decades of oil extraction by foreign energy firms.

But the unrest has as much to do with criminal profit from the industrial-scale theft of oil and kidnapping for ransom as it does with political struggle, and many Niger Delta residents say militancy has done little to improve their situation.