At least 200 people were killed in multiple raids in northwest Nigeria over the last week, authorities said Sunday, causing thousands to be displaced and dealing a heavy blow to authorities struggling to restore order.

Herders and farmers in Africa's most populous country have clashed over access to land in northwest and central Nigeria for years, but some groups have evolved into criminal gangs known as "bandits" who kill, loot and kidnap.

The minister of humanitarian affairs Sadiya Umar Farouq called the past week's attacks in Zamfara state "horrific and tragic," giving the first official toll after details began filtering out early Saturday.

"Over 200 persons were buried ... due to the invasion by bandits," she said in a statement confirmed by her spokesperson Nneka Ikem Anibeze on Sunday.

"We also worried about the displaced persons who are fleeing in their hundreds from their communities," the minister added, saying that relief material was sent to Zamfara.

The minister said more 10,000 people were displaced when "their homes were razed by the bandits while scores are still missing."

On Saturday, four residents had told Agence France-Presse (AFP) that gunmen rampaged through Anka and Bukkuyum districts over two days, killing at least 140 people.

Babandi Hamidu, a resident of Kurfa Danya village said the militants were shooting "anyone on sight."

Bello Matawalle, the governor of Zamfara state that borders Niger, downplayed the violence and said only 58 people were killed.

"The Emir of Anka gave a list of 22 deaths while the Emir of Bukkuyum gave a list of 36 victims," said a spokesperson for the governor, Zailani Bappa, in a statement late on Saturday.

Matawalle imposed highly restrictive measures for almost three months last year, shutting down telecoms, capping fuel sales and closing cattle markets in an attempt to weaken the bandits, but attacks have continued.

Revenge attacks?

Nigeria's President Muhammadu Buhari on Saturday condemned the attacks as "an act of desperation by mass murderers."

"We are fiercely determined to smoke out and destroy these outlaws," said the 79-year-old former army general, who is also battling a more than decadelong terrorist insurgency in the northeast and separatist agitation in the southeast.

On Wednesday, the government officially labeled bandits as terrorists, saying it would help bring tougher sanctions, but analysts say the move is to enable the military to deploy more resources to the northwest.

Bandits made international headlines last year after kidnapping hundreds of students in a series of mass abductions from schools and colleges.

Students are often quickly released after ransom payments to the bandits but 200 were still missing in September, according to the U.N.

On Saturday, 30 students and a teacher were released after seven months of captivity in Kebbi state, a local official said. It was not clear if any ransom had been paid.

Military and police operations are ongoing in the northwest.

Nigeria's armed forces said they had killed 537 "armed bandits and other criminal elements" in the region and arrested 374 others since May last year, while 452 "kidnapped civilians were rescued."

Security analyst Kabir Adamu, with Abuja-based Beacon Consulting Nigeria, told AFP that this week's raids could be in response to recent military operations.

Last month, bandits loyal to notorious gang leader Bello Turji suffered heavy losses in ground and air raids on their forest camps.

"Angered by this, and perhaps by the fact that that they were facing certain death, (they) decided to move to other locations and in the course of this they seem to be conducting these attacks," Adamu told AFP.

Locals said the raids were in response to an attack by vigilantes on a convoy of bandits who were trying to relocate elsewhere in the state.