At least 55 people were killed overnight in two attacks on villages in eastern Congo, the United Nations said on Monday, in potentially the worst night of violence the area has seen in at least four years.

The army and a local civil rights group blamed the Allied Democratic Forces (ADF), an Islamist armed group, for raiding the village of Tchabi and a camp for displaced people near Boga, another village. Both are close to the border of Uganda.

Houses were burned and civilians abducted, the U.N. office for humanitarian affairs said in a statement.

Albert Basegu, the head of a civil rights group in Boga, told Reuters by telephone that he had been alerted to the attack by the sound of cries at a neighbour's house.

"When I got there I found that the attackers had already killed an Anglican pastor and his daughter was also seriously wounded," Basegu said.

The Kivu Security Tracker (KST), which has mapped unrest in restive eastern Congo since June 2017, said on Twitter the wife of a local chief was among the dead. It did not attribute blame for the killings.

"It's the deadliest day ever recorded by the KST," said Pierre Boisselet, the research group's coordinator.

The ADF is believed to have killed more than 850 people in 2020, according to the United Nations, in a spate of reprisal attacks on civilians after the army began operations against it the year before.

In March the United States labelled the ADF a foreign terrorist organisation. The group has in the past proclaimed allegiance to Islamic State, although the United Nations says evidence linking it to other Islamist militant networks is scant.

President Felix Tshisekedi declared a state of siege in Congo's North Kivu and Ituri provinces on May 1 in an attempt to curb increasing attacks by militant groups.

Uganda announced earlier this month that it had agreed to share intelligence and coordinate operations against the rebels but that it would not be deploying troops in Congo.