Nigerian President Goodluck Jonathan has condemned Friday's suicide bombing attack on a United Nations building in the capital, Abuja, that left 18 people dead.

Jonathan described the bombing as "barbaric, senseless and cowardly" and ordered increased security around the capital following the blast.

The group Boko Haram, which wants Islamic sharia law implemented in the country, claimed responsibility for the attack in which a suicide bomber forced his way through two security gates and rammed his car into the building, blowing out the first two floors.

The building is located in the same neighbourhood as the US embassy and other diplomatic posts in Abuja.

Al Jazeera's Yvonne Ndege, reporting from the scene of the attack, said:"This is one of the heavily guarded buildings in Nigeria - it houses international diplomats and foreign workers.

"This entire area has been cordoned off. There are military men and security services in the area, security has been heightened within the entire city. Road blocks have been erected, they want to make sure there are no further attacks here in Abuja."

'Increasing threat'

In recent months, the country has faced an increasing threat from Boko Haram, which has been waging war against the Nigerian government, our correspondent said.

But she said violence in the past has centred on Nigerian symbols of authority and not foreign targets.

The group has previously carried out assassinations and bombings, including the June car bombing of the national headquarters of Nigeria's federal police that killed at least two people.

"The Islamist sect has in the past attacked symbols of authority so people are asking if this a change of tactic - with international people working for the betterment of Nigeria [being attacked]," our correspondent said.

The headquarters, known as UN House, had offices for about 400 employees working for 26 UN humanitarian and development agencies.

UN Secretary-General Ban Ki-moon called Friday's bombing "an assault on those who devote their lives to helping others."

"We condemn this terrible act, utterly ... We do not yet have precise casualty figures but they are likely to be considerable. A number of people are dead; many more are wounded,'' he said.

Ban said he was sending his deputy, Asha-Rose Migiro, to Nigeria immediately to meet officials in Abuja.

Source: Al Jazeera and agencies