Congo's war tops the agenda at this weekend's African Union summit but chances of diplomatic progress are slim as rebels advance and Rwanda's leader, accused of backing them, has vowed to "spit in the face" of anyone punishing him for his actions.

The advance of M23 rebels through eastern Congo has triggered fears that Africa's Great Lakes region risks slipping back into a broader war sucking in neighbouring armies, reminiscent of conflicts in the 1990s and 2000s that killed millions.

The AU summit in Addis Ababa on Saturday and Sunday is meant to be the next stage in regional efforts to halt the conflict. But rebels ignored calls for peace at a summit last week and have pushed towards Bukavu, some 200 km south of Goma, eastern Congo's biggest city which was seized last month.

"I think this is the most serious crisis in Africa right now. It is time for the African Union to show its strength," Stephanie Wolters, senior research fellow at the South African Institute of International Affairs, told Reuters.

World leaders have called for Rwanda to cut its support for the M23 fighters, which UN experts say includes troops and advanced military hardware.

The United States has warned of possible sanctions against Rwandan and Congolese officials. The European Parliament on Thursday urged the European Union to freeze direct budget support for Rwanda.

But President Paul Kagame remains defiant.

Rwanda has repeatedly denied backing M23 but says it will do everything necessary to defend itself from Hutu militants in Congo that Kigali, and M23, says remain bent killing Tutsis and attacking Rwanda 30 years after the 1994 genocide.

'EXISTENTIAL THREATS'

M23 is the latest in a string of Rwanda-backed rebel movements in Congo to cite this threat as they take up arms in the eastern borderlands, which are far from the capital, Kinshasa, and rich in minerals.

"We have faced existential threats for years. We had the worst tragedy in '94. You really come to me and start threatening ... sanctions because I am defending myself? And you think I have any iota of fear for that?" Kagame told France's Jeune Afrique magazine this week.

He recalled an episode from the genocide, when he said a woman about to be killed was told to choose how she would prefer to be killed. Instead, he said, she spat in the face of her assailants.

"You can imagine how much I am going to spit in the face of anyone who tells me to choose between which death I should face," he said.

Congo's President Felix Tshisekedi travelled to Germany on Thursday to take part in the Munich Security Conference, where he would seek greater international involvement resolving Congo's crisis, his office said in a post on X.

He is due to continue to the AU summit afterwards, a source in the presidency said.

M23 rebels last seized Goma in 2012. It only briefly held the city before withdrawing under intense international pressure, threats of aid cuts to Rwanda and an aggressive African force defeated the rebels.

This time, though, experts say there is more reluctance to take on Rwanda and the multitude of other global crises is eating into world leaders' attention.

The M23 advance south has been slowed by Congolese forces backed by thousands of Burundian troops, which have also deployed into Congo. Tensions between Burundi and Rwanda run high, adding to regional tensions.