Abuja - Nigeria's capital was on alert on Wednesday after an explosion went off outside a popular shopping centre, in the latest attack likely to be blamed on Boko Haram Islamists.
The first explosion happened at about 21:00 at a shopping plaza in the city's Wuse II district, spokesperson for the National Emergency Management Agency (NEMA), Yushau A Shuaib, said in a statement.
Rescue workers then rushed to the scene and cordoned off the area, where an unexploded bomb was later discovered, an official said.
"When we were trying to find out what is happening, the anti-bomb squad discovered another one. They just detonated it," the head of NEMA's Abuja office, Ishaya Chonoko, told journalists at the scene.
"The good thing is that there was no report of human casualty," he added.
The area was swarming with rescue workers overnight, as security forces kept journalists hundreds of metres away.
When the cordon had been cleared early on Wednesday, an AFP reporter saw that the windows of shops adjacent to the Banex Plaza shopping mall had been shattered, but the main centre had not evidently been affected.
The plaza in a prominent commercial area of Nigeria's capital is popular with both Nigerians and foreigners.
A police statement called the blast "a low-level explosion" and said that "intensive" surveillance patrols were ongoing around the city that has been repeatedly attacked by Boko Haram.
A suicide bomb attack on UN headquarters in Abuja in August killed at least 25 people, while another at the Abuja office of one of the country's most prominent newspapers left four dead.
Most recently, on 22 June, a blast went off outside a nightclub in Abuja that shattered the windows of nearby buildings but caused no casualties.
No group has yet claimed the latest attack.
The US embassy in Nigeria issued an "emergency message to inform US citizens of potential threats against US installations during the July 4 holiday week".
The statement posted on the embassy website said the US government was working with Nigeria's security to boost security measures through the independence holiday.
In Abuja, Americans remain barred from visiting places of worship as well as nearby commercial establishments and must return to their homes by midnight, the statement said.
Boko Haram has repeatedly attacked churches, typically on Sundays and holidays.
Last month, Washington designated three Boko Haram leaders as global terrorists, and the group has previously threatened to strike American interests.
Much of Boko Haram's violence has been concentrated in northern Nigeria, particularly in the northeast where they are believed to be based.
But they have in recent months expanded the targets down to the centre of the country, where Nigeria's beleaguered capital is located.
Abuja residents and foreigners who visit regularly have since learned to live with security checks and queues of cars waiting to be searched at prominent spots.
Boko Haram has claimed attacks that have killed more than 1 000 people in Africa's most populous nation and top oil producer since the middle of 2009.
Last month, President Goodluck Jonathan fired his defence minister and national security, amid heightened criticism of what some term his inadequate response to the Boko Haram insurgency.