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By Felix Onuah and Anamesere Igboeroteonwu

ABUJA/ONITSHA, Nigeria, May 20 (Reuters) - Nigeria's President Muhammadu Buhari on Friday said he ordered a heightened military presence in the restive Niger Delta region to deal with a resurgence of attacks on oil and gas facilities, a day after yet another pipeline explosion.

British Foreign Minster Philip Hammond warned on Saturday military action would not end a wave of attacks in the southern swamps because it did not address rising anger among residents over poverty despite sitting on much of Nigeria's oil wealth.

The rise in attacks in the Delta in the last few weeks has driven Nigerian oil output to a more than 20-year low, worsening a drain on public finances.

A group calling itself the Niger Delta Avengers has claimed responsibility for several sophisticated attacks.

Speaking at a meeting with Shell's upstream head, Andrew Brown, Buhari said he had instructed the chief of naval staff to reorganise and strengthen the military Joint Task Force to deal with the militancy.

"We have to be very serious with the situation in the Niger Delta because it threatens the national economy," Buhari said in a statement.

"I assure you that everything possible will be done to protect personnel and oil assets in the region," he added.

Nigeria had several times announced army reinforcements to the Delta but diplomats said the military has achieved little as militants were operating in small groups and hiding in the hard-to-access swamps.

"Mr. Brown had appealed for an urgent solution to rising crime and militancy in the Niger Delta," the presidency said.

An industry source told Reuters that major oil firms warned Vice President Yemi Osinbajo this month that a military crackdown was actually fuelling dissent in the Delta.

The presidency statement also quoted Brown as saying Shell would not pull out of Nigeria despite the violence and that it was in talks with state energy firm NNPC for new oil and gas projects.

Their was no immediate comment from Shell, but its country chair said in an interview published on Sunday the firm was committed to long-term investment in the West African nation.

Buhari's comments came after locals said a gas pipeline operated by NNPC was attacked late on Thursday.

The pipeline, which connects the Escravos oil terminal to Warri, supplies gas to different parts of the country.

Eric Omare, a spokesman for the Ijaw Youth Council, a youth umbrella, said the attack occurred near the village of Ogbe Ijoh, near Warri, "on the pipeline belonging to NNPC.