Nigerian President Umaru Yar'Adua has ordered the country's armed forces to tighten security in the Niger Delta and hunt down militants behind an attack on Shell's main offshore oil facility, his office said on Friday.

"The president has . directed that security be beefed up at all oil facilities and installations in the Niger Delta to forestall further acts of terrorism by criminal elements in the region," his office said in a statement.

Nigeria's armed forces and security agencies had been told to take "all necessary action" to apprehend the militants who attacked Royal Dutch Shell's Bonga oilfield, which lies some 120 km (75 miles) offshore, in the early hours of Thursday.

The attack forced the Anglo-Dutch giant to stop production at Bonga, cutting Nigeria's oil output by a tenth and shocking an industry that thought such deepwater sites in Nigeria were relatively immune from sabotage.

Shell on Friday declared force majeure on oil shipments for June and July from Bonga, which has a nameplate capacity of 220,000 barrels per day, meaning it cannot guarantee to meet its contractual obligations.

The group that claimed responsibility for the attack - the Movement for the Emancipation of the Niger Delta (MEND) - shrugged off the president's order over security.

"Empty talk from the commander-in-chief of an inept armed forces," it said in an email to Reuters.

MEND has mostly limited its strikes to bombing oil pipelines and kidnapping oil workers at onshore facilities in Nigeria.

It warned after the Bonga attack that oil and gas tankers in Nigerian waters may become targets, raising the prospect of a new campaign of sabotage in the waters of the Gulf of Guinea.

Nigeria's House of Representatives has called an emergency meeting for Monday with the defence and oil ministers, national security adviser and foreign oil firms to discuss the attack.

DARING STRIKE

Security experts said the Bonga attack was a coup for MEND, knocking out a significant portion of Nigerian output at a time when oil prices are highly sensitive to supply disruptions and marking their first significant strike deep offshore.

It also came as President Yar'Adua's administration prepares for a peace summit next month meant to address the underlying causes of the unrest in the Niger Delta.

Politicians, local chiefs and splinter militia groups are expected to attend but MEND and another group, the Ijaw Youth Council, have said they will not take part.

"Militants . who continue to spurn the peace overtures of the federal government must be prepared to face the full consequence of taking up arms against their fatherland," the presidency statement said.

Security analysts say those behind the attack on Bonga may have been a small group of well-trained fighters who have only a loose attachment to the fragmented leadership of MEND itself.