Rwanda-backed M23 rebels are moving “rapidly” through South Kivu province toward the city of Bukavu in eastern Democratic Republic of Congo, the United Nations (UN) said.

The UN is “deeply concerned” by the rebel advance, according to Stephane Dujarric, spokesperson for UN Secretary-General Antonio Guterres.

“Our colleagues in the peacekeeping mission say there are reports of Rwanda Defence Force cross-border movements in that direction” as well, Dujarric told reporters in New York on Thursday. Clashes between the rebels and Congolese troops are ongoing in the province, he said.

Bukavu is located along Congo’s border with Rwanda at the southern-most point of Lake Kivu. It’s a major trading hub and important transit point for minerals including gold.

M23 took the city of Goma in Congo’s mineral-rich North Kivu province earlier this week, backed by Rwandan soldiers, according to UN officials. They overpowered Congo’s army, UN peacekeepers and allied forces and now control the city of 2-million people.

Occasional shooting continues in Goma, which is littered with unexploded ordnance and has limited access to water and power, hampering humanitarian efforts, Dujarric said. Goma’s airport runway was also damaged in the fighting and isn’t operating, he said, an impediment to bringing in supplies.

Earlier this week, the US warned the Security Council that Rwanda and the M23 seemed intent to expand their conquest to South Kivu.

Regional War

UN peacekeepers withdrew from the province in June at Congo’s request, slowly drawing down a mission that began more than 25 years ago at the height of a series of wars that left millions of people dead.

South Kivu also borders Burundi, and troops from that country are fighting alongside the Congolese army against the M23, threatening to turn the conflict into a regional war, the International Crisis Group warned in a statement Tuesday.

The ICG said there’s “a real risk” that Rwanda may try to topple the Burundian government next, citing “regional diplomats.”

The M23 says it’s protecting the rights of Congolese ethnic Tutsis and other speakers of the Rwandan language in the country, as well as fighting a rebel group in Congo with links to the perpetrators of the 1994 genocide in Rwanda, which killed more than 800 000 people, mainly Tutsis.

Congo’s government maintains the M23 and Rwanda are after extensive mineral wealth in the east. Rwanda denies backing the rebellion or sending troops into the country.