The United Nations Security Council has renewed the mandate of the peacekeeping force in the Democratic Republic of Congo but decided to cut its numbers.

The 15-member council on Friday unanimously renewed the $1.2bn mission for another year amid warnings that violence was spreading across the country before elections this year.

The French-drafted resolution reduced the authorised size of the military component of the MONUSCO mission from 19,815 to 16,215 troops, but the force is already under-strength and, in practice, fewer than 500 will go home.

The document is the first adopted since the United States began a review of the 16 UN peacekeeping missions as part of its plan to cut foreign aid and reform the world body's operations.

The resolution authorises the replacement of some troops with better-trained specialised units and gives the force a green light to intervene anywhere in the country if needed and not just in the volatile east.

It also calls for a dialogue between the UN and the Congolese government, led by President Joseph Kabila, on an "exit strategy".

Support of the resolution by all 15 council members reflects widespread backing for the US goal of streamlining the UN's operations that deploy over 107,000 troops and civilians at an annual cost of more than $7.8bn.

MONUSCO is the biggest and costliest UN mission with about 22,400 people, including nearly 17,000 soldiers and over 1,350 police.

Nikki Haley, the US ambassador to the UN, told the council she was "very proud" that all members had voted to cut the troop ceiling and make the mission stronger and more effective.

The UN force is going "to make sure the elections are safe, to make sure that we're keeping people safe, and that we're actually looking at the political problems on the ground", she said.

Potential war crime

At the start of Friday's council meeting, members stood in silent tribute to two UN experts killed while investigating alleged human rights violations by Congolese forces and local militia groups and all other victims of violence in the country.

The bodies of American Michael Sharp, Swedish national Zaida Catalan and their interpreter, Betu Tshintela, were found earlier this week - but three other Congolese members of their team remain missing.

READ MORE: Bodies of missing UN workers found in DRC

Recent acts of violence, including the killing of the experts, could constitute war crimes, the International Criminal Court's prosecutor said on Friday.

"There have been reports of violent clashes between local militia and Congolese forces, a large number of killings of both civilians and non-civilians, kidnappings and summary executions," Fatou Bensouda wrote in a statement.

"Such acts could constitute crimes within the jurisdiction of the International Criminal Court."

The remote central province of Kasai has been plagued by violence since mid-August, when government forces killed Kamwina Nsapu, a tribal chief and militia leader who had rebelled against Kabila's central government.

The violence has spilled over from Kasai to the neighbouring provinces of Kasai-Oriental and Lomami, leaving at least 400 people dead.

UN members are pressing Kabila's government to honour a power-sharing deal with the opposition before elections later this year.