A suspected female suicide bomber has killed at least 17 people in Maiduguri, capital of Nigeria's Borno state, military and hospital sources say.

Tuesday's attack happened three days after a multiple bomb attack in the city killed more than 50 people.

Citing a military source, the Reuters news agency said a loud blast rocked Maiduguri just after 4pm local time (1500 GMT) and that the bomber detonated her device at a roundabout near the Monday Market, which has seen numerous attacks before.

Maiduguri is the birthplace of Boko Haram, which has been fighting for six years to enforce Islamic law in all 36 states of Nigeria, which is roughly equally divided between a predominantly Muslim north and a mainly Christian south.

The latest attack came after the government in Abuja described the fighters' pledge of allegiance to the Islamic State in Iraq and the Levant (ISIL) as a sign of weakness in the face of growing military pressure from Nigeria and its allies.

Mike Omeri, the national security spokesman, called it "an act of desperation and comes at a time when Boko Haram is suffering heavy losses".

Witness account

Boko Haram fighters tried to seize Maiduguri at the end of January, killing more than 100 people in the attack, and again in early February. 

A source at the State Specialist Hospital said 12 bodies had been brought to the morgue.

Another bomb was discovered and defused by police on Tuesday in Babalayi, a densely populated district of Maiduguri and about 500 meters from the scene of Thursday's explosion, a member of the civilian joint taskforce said.

A police spokesman did not immediately respond to requests for comment.

The city of about two million people was hit by several bombs on Saturday, one of them inside the Monday Market, in an attack that bore the hallmarks of Boko Haram.

Source: Agencies