Islamic State has claimed responsibility for an attack on the convoy of a state governor who was headed to a rally in northeastern Nigeria ahead of Saturday's presidential election.
The group said in a statement on Wednesday that 42 people were killed in Tuesday's attack on Borno state's governor.
But official sources told Reuters between three and 10 people were killed, and that some of them may have been beheaded.
Boko Haram has waged a decade-long insurgency in Nigeria's northeast, which has killed around 30,000 people and forced two million to leave their homes.
Islamic State West Africa Province, which split from Boko Haram in 2016, has carried out a series of attacks on military targets in recent months.
Security sources said the gunmen opened fire at the motorcade transporting Borno's state governor Kashim Shettima on his way from state capital Maiduguri to the market town of Gamboru for a rally.
There was no indication of how Shettima, a government politician, was affected by the attack. His spokesman could not be reached for comment.
Incumbent Muhammadu Buhari is seeking a second term in office in Saturday's elections in Nigeria, Africa's biggest democracy. He faces a tight contest against his main challenger, Atiku Abubakar, a businessman and former vice president.