An explosion has struck a busy shopping district in Nigeria's capital, Abuja, killing at least 21 people.

The explosion was in the capital's Wuse district, near the popular Banex plaza shopping complex, and could be heard from miles away.

It is not yet clear what was behind the explosion, however police say they are securing a "crime scene".

Islamist militant group Boko Haram has bombed targets across northern Nigeria in recent years.

"After a preliminary investigation we can confirm that 21 people were killed and 17 injured," police spokesman Frank Mba told reporters.

'Covered in blood'

Chiamaka Oham, who was near the site of the blast, told the BBC: "We heard a really loud noise and the building shook, and people started screaming and running out.

"We saw the smoke and people covered in blood. It was just chaos."

Eyewitnesses at the scene described seeing body parts scattered across the area.

The area was packed with shoppers at the time of the blast, the BBC's Hausa service editor Mansur Liman reports.

Many cars outside the shopping complex were burnt out and many windows were shattered, he adds.

One man told the BBC his driver was killed in the blast: "I was in the complex when I saw that the ground was shaking. I saw my driver dead and a lot of casualties."

Manzo Ezekiel, spokesman for the National Emergency Management Agency, told AFP news agency: "You can see smoke billowing from the sky. It's a very crowded place."

Police spokesman Frank Mba said he could not provide details of the nature or extent of the damage.

"Our most important assignment now is to secure lives, secure the crime scene and actually carry out preliminary investigations," he added.

Boko Haram has staged previous attacks in Abuja, but most of its targets have been in the north-east of the country.

In April, more than 70 people were killed in a bomb blast at a bus stop on the outskirts of the capital in an attack claimed by Boko Haram.

The group also said it was behind a car bomb attack near a bus station in the suburbs in May, which killed at least 19 people and injured 60 others.

The group also carried out a deadly car bomb attack on the United Nations building in the Nigerian capital in 2011.

It has become a source of growing international concern since the recent abduction of more than 200 girls from a school in northern Nigeria.

Who are Boko Haram?

• Founded in 2002
• Initially focused on opposing Western education - Boko Haram means "Western education is forbidden" in the Hausa language
• Launched military operations in 2009 to create Islamic state
• Thousands killed, mostly in north-eastern Nigeria - also attacks on police and UN headquarters in capital, Abuja
• Some three million people affected
• Declared terrorist group by US in 2013