Democratic Republic of Congo's M23 guerrillas, including rebel supremo Sultani Makenga, have surrendered en masse in Uganda, officers said Thursday, signalling the end of an 18-month insurgency.

The rebel surrender follows a crushing defeat at the hands of the UN-backed Congolese armed forces.

"He is with our forces, yes, Makenga has crossed into Uganda," a senior Ugandan military officer told AFP, although he declined to clarify if he had formally surrendered or was under arrest.

Paddy Ankunda, a colonel in the Ugandan army, told AFP that 1,500 men from the M23 -- a number thought to account for more or less the entire force -- had crossed into Uganda and given themselves up, and were now being held in the Kisoro border district.

"About 1,500 fighters surrendered today," said Ankunda, who is spokesman for Defence Minister Crispus Kiyonga, the mediator in stalled peace talks between M23 and Kinshasa. However, Ankunda said he was "not aware" if Makenga was among those to have surrendered.

Uganda has been accused by United Nations experts of backing the M23, claims Kampala has strongly denied.

The rebels' surrender puts paid to fears that they might try to fight on despite having been outweighed by superior firepower, notably helicopter gunships.

Makenga, 39, a former colonel in the DR Congo army, is accused of masterminding killings, abductions, using rape as a weapon of war and recruiting child soldiers, and is on both UN and US sanctions lists.

His prescence in Uganda -- arrested or not -- poses a diplomatic headache for Kampala.

Congolese troops backed by a special UN intervention brigade with an offensive mandate launched a major assault late last month against the M23 force of army mutineers in turbulent North Kivu.

The region is rich in natural resources, especially gold, coltan and tin, which have been fought over by a range of armed groups for the past 15 years.

The Movement of March 23 (M23) was founded by ethnic Tutsi former rebels who were incorporated into the Congolese army under a 2009 peace deal but mutinied in April 2012, claiming that the pact had never been fully implemented.

After briefly seizing the regional capital and mining hub of Goma last November, the M23 entered into fresh peace talks which fell apart last month, leading the Congolese army to go on the attack in a bid to end the rebellion.

US 'pressured' Rwanda to stop backing rebels

The M23 had been severely weakened by internal splits that degenerated into bloody fighting in March. The total withdrawal of Rwandan support proved the last straw.

Analysts say the United States distancing itself from Rwanda was decisive in the Rwandan government's decision to break off all links with the M23.

"It's clear that the US played a behind the scenes role in piling pressure onto Rwanda," said Richard Downie of the Centre for Strategic and International Studies in Washington.

Tanzania, which is one of three countries making up the UN intervention brigade, thanked the force for its work.

"Our forces did some good work. They carried out their mission with an outstanding level of professionalism," President Jakaya Kikwete told parliament.

The United Nations and rights groups have accused the M23 of atrocities including rape and murder in a conflict that caused tens of thousands of refugees to flee.

Makenga was born to parents from the Masisi area north of Goma, but grew up in the neighbouring Rutshuru district.

Like many of the ethnic Tutsi officers who fought alongside him, he cut his teeth in the ranks of the Rwandan Patriotic Front, now in power in neighbouring Rwanda, when it launched a rebellion in the early 1990s.

He then served as a battalion commander in the Rwandan-backed Congolese Rally for Democracy before joining former DRC rebel leader Laurent Nkunda's National Congress for the Defence of the People.

Ever since he has been seen as loyal to Nkunda, who has spent the past several years under house arrest in Rwanda after he fell out with his former mentors in Kigali.