Maiduguri - Boko Haram on Thursday claimed a suicide bomb attack that killed two police officers in northeast Nigeria, just days before a regional security summit on efforts to eradicate the Islamists.
The group, using the name Islamic State West Africa Province (ISWAP), said in a statement posted on social media that the bomber "was able to detonate his explosive vest in the area of Maiduguri".
It claimed "at least 15 apostates (non-believers)" were killed in the attack, which happened at about 12:00 pm (1100 GMT) at the Borno State Secretariat in the city.
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But Nigerian Army spokesman Sani Usman said only two died and said the attacker blew up as security personnel stopped him from trying to get into the government offices.
"Unfortunately in the process of stopping him, he detonated the improvised explosive device on his body, instantly killing himself, a policeman and critically injuring another policeman.
"Sadly, the injured policeman died later," Usman said in an emailed statement, adding that 18 people were injured and taken for treatment.
Mohammed Kanar, regional coordinator for the National Emergency Management Agency, and an accident and emergency spokesman for the Borno State Specialist Hospital, also confirmed just two deaths.
Between 19 and 24 people were injured in the blast, they added.
Locals and street vendors earlier told AFP the explosion initially appeared to come from a passing motorised rickshaw, which went up in flames and was gutted in the blast.
Boko Haram suicide bombers have previously used public transport to travel to multiple targets in the northeast and wider north.
The Islamist group was founded in Maiduguri in 2002 and the city has been repeatedly attacked since the insurgency turned violent in 2009.
But a relative calm has returned there in recent months as a military counter-insurgency makes apparent gains against rebel strongholds across the northeast.
Nigeria's President Muhammadu Buhari is this weekend hosting leaders of neighbouring countries, plus French President Francois Hollande, and senior British and US government officials.
The meeting is expected to focus on boosting regional and international cooperation in defeating the Islamic State group affiliate, which has also attacked Cameroon, Chad and Niger.
Buhari's office said on Tuesday "the successful conclusion of ongoing military operations against Boko Haram" was "at the top of its agenda".
The last successful attack in Maiduguri was a double suicide bombing by two women at a mosque in Molai, on the outskirts of the city, in March in which 22 people were killed and 35 injured.
Boko Haram has previously targeted government buildings and infrastructure in the insurgency, which has claimed at least 20,000 lives and made more than 2.6 million homeless since 2009.
It has also used suicide bombers to inflict maximum civilian casualties in attacks on "soft" targets such as mosques, crowded market places and bus stations.
On Wednesday, the Nigerian military said troops and civilian vigilantes prevented a suicide bomb attack at a mosque in Maiduguri's Sulaimanti suburb.
Five civilians were injured during the attempt and bomb disposal specialists were called in to make safe the device the bomber was carrying, army spokesman Usman said in a statement.