LAGOS (AFP) - Nigerian militants said Wednesday they blew up two key oil pipelines as they stepped up attacks in response to a government amnesty offer.

The Movement for the Emancipation of the Niger Delta (MEND) said it had blown up pipelines operated by the Anglo-Dutch giant Shell and Italian group Agip in a pre-dawn attack in the Bayelsa state.

MEND, the best equipped of a number of rebel groups operating in the restive southern oil hub, has claimed at least seven attacks since the government's amnesty offer on June 25.

The militants say are on a campaign to cripple the oil industry until their complaints of injustices and inequitable distribution of the oil wealth are answered.

"The Agip pipeline which connects the Agip Brass terminal was sabotaged at Nembe creek while the Shell Nembe creek line was done at Asawo village, all in Bayelsa state," the militants said.

A Shell spokesperson told AFP "we are investigating reports of an attack on SPDC (Shell Petroleum Development Company) joint venture's Nembe Creek at Bayelsa."

Agip was not immediately available for comment.

Nigeria's oil unions on Wednesday urged oil workers to be more vigilant in their duty posts as militants stepped up violence in the volatile region despite a government amnesty.

The blue-collar National Union of Petroleum and Natural Gas Workers (NUPENG) and its white-collar counterpart PENGASSAN made the call in a statement published after a joint national executive council (NEC) meeting in Abuja.

"On the issue of security and safety of industry personnel and work locations, the joint NEC-in session advised its members to be more vigilant," they said.

Both unions said they hoped a government amnesty offer to rebels in the region would "yield the desired results."

President Umaru Yar'Adua last month declared an unconditional pardon for armed groups who say they are fighting for a greater share of the country's oil wealth for the local people of the Niger Delta.

Although the amnesty offer remains open until October 4, the insurgence has persisted.

MEND claimed Wednesday that some of the armed youths who responded to the amnesty offer, have been arrested by a special joint military and police unit deployed in the region to quell the violence.

The violence in the Niger Delta has cost Nigeria hundreds of thousands of barrels of crude each day. Coupled with global crude price fluctuations, the toll on Nigeria's economy has been substantial.

Lamido Sanusi, the central bank chief of Nigeria, the world's eighth oil exporter, on Tuesday said the country's foreign exchange reserves had plummeted by about 10 billion dollars in six months to 43.19 billion dollars as of last week.

Africa's most populous country relies on oil for more than 90 percent of its export earnings.

The unrest has cut Nigeria's crude output by around 30 percent from around 2.6 million barrels per day in 2006 to 1.8 million now.

Nigeria was Africa's leading oil producer but it is currently neck-and-neck with Angola since the troubles in the Niger Delta started.

Militant attacks in the region have targeted Shell, Agip and the American group Chevron. Hundreds of workers in the oil industry and its support sectors have also been kidnapped.

This week MEND abducted six foreign workers -- two Russians, two Filipinos, a Ukranian and an Indian national -- crew members of a chemical tanker.