Armed Congolese rebels crossed back overnight into the country from their longtime refuge in Uganda, prompting fears that a guerilla conflict put to rest in 2013 could be rekindled.

At least 200 former members of M23, an ethnic Tutsi group defeated by the Congolese army three years ago, arrived from Uganda and took over a village in North Kivu province, Democratic Republic of Congo government spokesman Lambert Mende told AFP.

Mende said the Congolese army was fighting two battalions installed in Ishasha village "who were supposed to be in Uganda under the responsibility of that country's authorities".

"How could our Ugandan neighbours, with whom we are bound by very serious commitments, allow people who had been living in refugee camps to cross over -- armed -- into our territory?" he added.

Kinshasa said a former army general, Sultani Makenga, was leading one of the battalions.

No military spokesperson was available to confirm the clashes with M23. Omar Kavota, director of the Centre for the Promotion of Peace, Democracy and Human Rights said Sunday that sources in Ishasha had confirmed the M23 presence there but said "there hasn't been fighting or clashes yet".

- An 'old trick' -

Congo's resource-rich eastern provinces have suffered years of brutal conflict, with neighbouring states backing rebel groups in a civil war against Kinshasa's authority, and roaming armed militia triggering the mass flight of terrorised civilians.

After its November 2013 defeat at the hands of Congolese and UN forces, M23 agreed to a plan to disarm, demobilise and reintegrate its soldiers into DRC civilian life.

But the return of the former rebels has stalled, with fewer than 200 of the 1,900 sheltering in Uganda and only 13 out of hundreds left in Rwanda coming back.

During the civil war Congolese authorities denounced Rwanda and Uganda for allowing the rebel groups to use their territory as staging grounds for attacks. More recently, they have blamed the states for "bad faith" for allowing "criminals to circulate freely" instead of extraditing them to stand trial in DR Congo.

Uganda Foreign Minister Henry Okello Oryem, reached by AFP in Kampala, strongly denied backing the M23 move across the border.

"The fact that Uganda would arm or authorise M23 rebels to be based here (in Uganda) and destabilise the DRC is the fruit of their imagination. Why would Uganda arm the M23?" he said.

Kinshasa's central government would do well to "focus on the problems of governing its own country", the minister added.

Congo is in political limbo, as President Joseph Kabila has refused to step down after his second and final mandate ended in December.

A New Year's Eve deal calls for the appointment of a new prime minister and a transitional body to pave the way to elections in December 2017 -- but not all opposition groups have signed on.

A member of the country's political opposition coalition suggested the return of the M23 could be Kabila up to his "old tricks", fomenting unrest to detract from the political crisis in Kinshasa.

"Every time President Kabila loses ground on the political front, he reactivates armed groups with the goal of taking us back to the old situation" of instability, the opposition member told AFP, requesting anonymity.