Nigerian forces battled Boko Haram insurgents in the capital Abuja Friday as the military sent troops to a town where the Islamists slaughtered at least 87 people, casting doubt on claims that the group's rebellion has been contained.
Boko Haram fighters opened fire on security agents in the capital who -- acting on a tipoff from arrested insurgents -- were searching for a weapons cache purportedly hidden in an unfinished building.
They began "digging for arms" shortly after midnight, said Marilyn Ogar, spokeswoman for Nigeria's main intelligence branch.
They then "came under heavy gunfire attack by...Boko Haram elements within the area, which prompted immediate response from the security team," she said in a statement.
Several people were injured in the shootout and 12 were later arrested, according to Ogar.
Police in Abuja told AFP that a number of people were killed, but declined to provide figures.
The gun battle in the heart of the capital was seen a significant turn in Boko Haram's four-year insurgency, which the group has said is aimed at creating a strict Islamic state in the north.
The military launched a major offensive against Boko Haram four months ago in the northeast, the Islamists' stronghold, and has described the extremists as being in disarray.
Since the offensive began, most of Boko Haram's attacks have been concentrated in remote parts of the northeast, typically targeting vulnerable civilians.
The bloodshed in Abuja was the first recorded assault in the centre of the country in many months.
Past attacks in the capital have included the August 2011 bombing of a UN building that killed 25 people as well as bombings at newspaper offices and a shopping centre.
This week also saw the deadliest attack since the military offensive was launched.
Late Tuesday, in the remote northeastern town of Benisheik in Borno state, heavily armed Islamists disguised in military uniforms burnt scores of homes and buildings and killed dozens.
They stormed the area in some 20 trucks and were equiped with "anti-aircraft guns" according to a security source who requested anonymity.
Witnesses and army spokesman Ibrahim Attahiru said the security forces were overwhelmed by the Boko Haram assault, and some reported that military personnel abandoned the town amid the killing.
Survivors described a roadside littered with corpses.
Saidu Yakubu of Borno's Environmental Protection Agency told journalists who visited the town Thursday that 87 bodies had been discovered after the massacre, but that toll could rise.
An official with the same agency, Abdulaziz Kolomi, told reporters that rescue teams "did not go deep into the bush" in their search for victims.
"I strongly believe many people have fallen there," he said, raising the possibility that more bodies could still be found.
Army General Mohammed Yusuf told journalists who visited the area that a "full reinforcement" of Benisheik would take place Friday.
A state official, who asked for anonymity, said more troops had been sent Friday to Benisheik, which is roughly 70 kilometres (43 miles) from Borno's capital Maiduguri, where Boko Haram was founded more than a decade ago.
During its offensive, the military has claimed the killing of top Boko Haram commanders including the group's leader Abubakar Shekau, declared a global terrorist by the United States which put a $7 million bounty on his head.
But Shekau's death has not been independently verified and the military's repeated claim that Boko Haram is on the defensive is being viewed with increased scepticism.
In the last three months, dozens of school children as well as Muslim gathering for morning prayers have been among those slaughtered.
Vigilante groups, which formed in the northeast to help the military, have been repeatedly targeted in deadly attacks.
Despite living in Africa's top oil producer, most Nigerians survive on less than $2 a day and poverty, most acute in the north, is seen a key driver of the insurgency.
The southern half of the country is mainly Christian.
Tolls earlier this year said the Boko Haram conflict has cost more than 3,600 lives, but the current figure is likely much higher.