Fifteen people died Friday in twin car bomb blasts in Nigerian capital Abuja as the West African nation commemorated its 50th anniversary of independence from Britain, witnesses said, dpa reported.
The Movement for the Emancipation of the Niger Delta (MEND) warned in an email that it had planted bombs and gave people 30 minutes to begin evacuating Eagle Square in Abuja, where President Goodluck Jonathan was attending a celebration.
"I heard two loud explosions at short intervals, and I later saw at least 15 bodies being taken away in ambulances while other bodies were still lying there," a witness told the German Press Agency dpa.
Another witness said security had cordoned off the scene of the explosion, which took place just outside the square, while the event continued as planned.
More than 1,000 schoolchildren performed a colourful calisthenics display while charred corpses were being removed a few hundred metres away.
"Several explosive devices have been successfully planted in and around the venue by our operatives working inside the government security services," MEND spokesman Jomo Gbomo had said in the email warning.
"There is nothing worth celebrating after 50 years of failure," he added. "For 50 years, the people of the Niger Delta have had their land and resources stolen from them."
In Washington, US State Department spokesman PJ Crowley said the United States condemned the bombings and expressed condolences to the families of the victims.
"As Nigeria casts its eyes forward to new elections and a reaffirmation of democratic values, it is unfortunate that there remain those who would resort to violence at this critical time," he said in a statement. "Violence has no place in political discourse."
The anniversary had caused introspection in Africa's most-populous nation, which has seen civil war, military dictatorships, rigged elections and endemic corruption in its 50 years of independence.
A previous attack by the group in March saw two car bombs detonate outside peace talks aimed at ending years of unrest in the oil- producing Niger Delta.
MEND drastically cut Nigeria's oil output through years of sabotage of oil facilities in the Niger Delta, saying it was fighting for a more equitable share of resources for indigenous peoples.
A government amnesty last year saw many militants hand in their arms, leading to a huge decrease in militant activity in the area.
The bomb attack will raise fears that MEND is planning a return to full-scale violence, and could once again cut the output of Africa's largest oil producer, which has risen from about 1.6 million barrels per day pre-amnesty to roughly 2 million barrels.