An explosion at a United Nations office complex in Nigeria's capital, Abuja, could have killed at least 18 people and injured more than 60, officials said.

The explosion took place Friday after a vehicle penetrated the protective barriers at the compound and crashed into the main UN building on Independence Avenue in Abuja's central business district, Manzo Ezekiel, a spokesman for the National Emergency Management Agency, said at the scene.

The city's Police Commissioner, Mike Zuokumor, told reporters at least 18 people were killed in the attack.

The explosion, which levelled one wing of the three-storey building, is thought to have left others trapped in the rubble. One UN staff member, who did not wish to give her name, said: "I don't know what is going on. Many people are still trapped upstairs and we need a crane to bring people down."

The UN building is located in Abuja's diplomatic zone, not far from the US embassy. It houses about 400 employees of the UN in Nigeria and the majority of its offices.

Witnesses at the city's National Hospital said they saw more than a dozen victims arrive. ''The hospital has suspended normal services to attend to the victims, and we, the patients, were asked to go,'' Jessica Effiong, a witness, said by phone.

But a police spokesman, Yemi Ajayi, could not confirm exact numbers. "We don't have exact figures yet and we don't want to guess," he said.

No group has claimed responsibility for the blast but there is speculation it could have been carried out by a militant Islamic group known as Boko Haram, which draws inspiration from Afghanistan's Taliban movement.

Nigerian authorities have blamed Boko Haram for a series of bomb attacks. It claimed responsibility for a June 16 blast at the police headquarters in Abuja that left five people dead and 11 injured. But the oil-rich country faces terrorist threats on multiple fronts. Last year, a militant group from the country's oil-producing Niger Delta detonated car bombs in the capital during Nigeria's 50th independence anniversary ceremony, killing at least 12.

Nigeria is roughly split between a mainly Muslim north and a predominantly Christian south. More than 14,000 people died in ethnic and religious clashes in the West African nation between 1999 and 2009, according to the Brussels-based International Crisis Group