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LAGOS, Nigeria (AP) — At least 23 travelers have been killed in northwest Nigeria in an attack by the same armed groups blamed for killing thousands this year in Africa’s most populous country.

Nigerian President Muhammadu Buhari said he is “very distressed” over the attack that occurred Monday though details of the incident emerged only on Wednesday. It is the latest in a cycle of violence targeting rural communities and travelers along highways in the northwest and central parts of the West African nation.

Sokoto state governor Aminu Tambuwal said that the gunmen opened fire at a bus conveying the travelers along a route notorious for such attacks in the Isa area of the state. Twenty-three of the travelers died of fire burns while six others were injured, he said.

No group has claimed responsibility for the attack but blame quickly fell on assailants who have killed at least 2,500 people in the northwest and central states in the first half of 20201, according to data from the U.S. Council on Foreign Relations.

Buhari — who was elected in 2015 after promising to crush Islamist extremists in the northeast — has seen the armed violence in other parts of northern Nigeria grow rapidly under his watch. He faces mounting pressure to act. On Wednesday, he said the latest attack “shows that the evil this administration is confronting is one that requires the support and involvement of all Nigerians.”

“I extend deep condolences to the families of the victims as I assure that the security agencies will continue to give their all to bring to an end the operations of these despicable people,” Buhari said.

The armed groups often target communities where they outnumber and outgun security operatives. They mostly consist of young men from the Fulani ethnic group, who had traditionally worked as nomadic cattle herders but are caught up in a decades-long conflict with Hausa farming communities over access to water and grazing land.

In late November, Nigeria designated them a terrorist organization and authorities promised that will create new chances to prosecute the gunmen, who are rarely charged in court.

In condemning the latest attack in Sokoto, where nearly 100 people have been killed in the last three months, state police commissioner Kamaldeen Okunola promised that “strategies are on to make sure that this will not happen again.”