Gunmen from a radical Muslim sect attacked a town hall yesterday in rural northeast Nigeria, killing at least 20 people who had gathered for a meeting of traders who are members of a Christian ethnic group, police said.

The attack at noon targeted a town hall where Christian Igbo people were holding a meeting, with gunmen chanting “God is great’’ as they fired Kalashnikov rifles.

The killings claimed by Boko Haram came after the group threatened to begin targeting Christians living in the country’s Muslim north in its increasingly bloody sectarian fight with Nigeria’s weak central government. That could further inflame religious tensions in a nation gripped by civil unrest.

The attack occurred in Mubi in Adamawa state as Igbo traders held a meeting before opening up their shops for business, local police commissioner Ade T. Shinaba said.

“We started hearing many gunshots through the windows,’’ said Okey Raymond, 48, who attended the meeting. “Everyone scampered for safety, but the gunmen chanted ‘God is great, God is great’ while shooting at us.’’

Raymond said he hid under a table and escaped out a back door. The gunmen also carried knives and machetes, the police commissioner said.

No arrests have been made in the attack, which left at least 15 people wounded.

Gunmen also attacked a church in the northeast Nigeria city of Gombe during a prayer service Thursday night, spraying congregants with gunfire and killing at least eight people, local medical officials said.

In a statement yesterday to The Daily Trust newspaper, a Boko Haram spokesman claimed responsibility for the attacks in Gombe and Mubi.

“We want to prove to the federal government of Nigeria that we can always change our tactics,’’ he said.