Rebels from Nigeria's oil-producing Niger Delta said they had blown up a Royal Dutch Shell pipeline and killed 11 soldiers in a firefight on Monday, but the army denied losing any men.

The rebel Movement for the Emancipation of the Niger Delta (MEND) said in an emailed statement that it had sabotaged the Shell pipeline at Awoba flow station in southern Rivers state in the early hours of Monday morning.

"Today's attack is dedicated to the administration of (President) Umaru Yar'Adua and (Vice President) Goodluck Jonathan who have failed after one year in office to ensure peace, security and reconciliation in the Niger Delta region," the MEND statement said.

Shell in Nigeria said it was investigating and had no immediate comment.

Nigeria's army initially denied there had been an attack but later confirmed there had been a blast early on Monday at a Shell facility close to Awoba which was not guarded by soldiers.

"The cause of the explosion has not yet been determined but it is strongly suspected that explosives might have been used by miscreants," Sagir Musa, military spokesman in Rivers state, told Reuters.

Musa said troops had been sent to the site of the blast but denied that 11 soldiers in a military gunboat had been killed in a shoot-out.

The Niger Delta is home to the world's eighth-biggest oil industry, exporting about 2.1 million barrels per day, but rebels have led a campaign of sabotage since early 2006 to push demands for greater local control over oil revenues.

The unrest has depressed Nigerian output by around a fifth since then, helping to push world oil prices to record highs. Oil rose above $133 a barrel on Monday after striking a record high of $135.09 last week.

A new government in Nigeria led by Yar'Adua and Jonathan, a native of the delta, took office on May 29 last year promising to address the root causes of the violence in the Niger Delta and to negotiate with the militants.

But attacks on oil installations and the kidnapping of oil industry workers have continued in the region, with MEND last week accusing the government of "insincerity" in its handling of the situation.

Shell was forced to shut in about 164,000 barrels per day of Bonny Light crude production -- or about 40 percent of the Anglo-Dutch major's equity oil output in Nigeria -- late last month due to militant attacks in the delta.

The company has been restoring some of the shut-in production but a force majeure remains in place for Bonny Light exports, meaning it cannot guarantee to meet its contract commitments.