Republic of Congo's foreign minister has called on the US President Donald Trump's administration to expand targeted sanctions against Rwanda to "restore the credibility" of its mediation efforts, as Rwanda-backed M23 rebels advanced on a strategic town bordering Burundi.

The United Nations says some 200 000 people have fled their homes in recent days and scores of civilians have been killed, casting further doubt on hopes for an end to violence in eastern Congo.

Just last week Congolese President Felix Tshisekedi and his Rwandan counterpart Paul Kagame met with President Donald Trump in Washington and affirmed their commitment to a US-brokered peace deal.

In an interview with Reuters late on Tuesday, Foreign Minister Therese Kayikwamba Wagner said Washington should ramp up sanctions on Kigali in light of the violence.

She proposed measures targeting "individuals in the chain of command" and institutions like the Rwandan military that could hurt Kigali's ability to purchase weapons.

"Washington has to restore the credibility of its process through accountability," Wagner said.

"It is not enough to condemn. It is not enough to be preoccupied or to be concerned. We're counting deaths in the thousands and concern is not the issue here."

Rwanda denies backing M23 in Congo, saying its troops are in eastern Congo for "defensive measures," although Washington and the United Nations say evidence of Rwandan support for the rebels is clear.

Rwanda's foreign minister, Olivier Nduhungirehe, told Reuters on Wednesday that more sanctions would not end the fighting. He blamed the recent escalation in fighting on a lack of political will by Kinshasa to implement peace deals or honour a moratorium on airstrikes that he said was agreed at a meeting in Washington last month.

Congolese forces have been attacking rebel positions and local communities in South Kivu province, where Uvira is located, for "weeks and months" before the recent escalation, he said.

The US, Britain and the European Union have already imposed sanctions on several M23 and Rwandan officials and some of their businesses over the rebellion.

Diplomats say Washington has another package of sanctions ready if Rwanda fails to meet commitments made in US-brokered deals to lift what Kigali calls its defensive measures in eastern Congo by the end of the year. 

REGIONAL VIOLENCE FEARED

M23 staged a lightning advance in eastern Congo this year, seizing more territory than ever before, including the region's two largest cities. The fighting has killed thousands and displaced hundreds of thousands more.

Even as the group takes part in peace talks led by Qatar in Doha, the group is tightening its grip on power.

Reuters reported on Monday that M23 had captured Luvungi, a town that had stood as the front line since February, and that fierce fighting was under way near Sange and Kiliba, villages further along the road towards Uvira from the north.

In a statement on Wednesday, Rwanda's foreign ministry accused the Congolese and Burundian militaries of escalating the violence in eastern Congo and said Kinshasa was never serious about peace negotiations mediated by Washington and Qatar.

"The international community has not demanded an end to these attacks prepared for months by the DRC and instigated in the past week," it said.

On Tuesday the US and nine other members of the International Contact Group (ICG) for the Great Lakes voiced "profound concern" over the renewed clashes, warning that violence could destabilise the wider region.

Burundi's foreign minister, Edouard Bizimana, told Radio France Internationale on Wednesday that his country had registered more than 30 000 refugees from Congo in the past three days.