The United States, France and Britain pressured Rwanda on Sunday over its support for rebels rapidly closing in on eastern Congo's largest city, as government forces struggled to halt an offensive that has forced thousands of civilians to flee.

The Democratic Republic of the Congo has severed diplomatic relations with Rwanda. The decision is based on allegations that authorities in Kigali are supporting rebels attacking Congolese government forces.

Rebels in Democratic Republic of Congo are closing in on a key city in the mineral-rich region aided by neighboring Rwanda’s army, the head of a peacekeeping mission told United Nations Security Council members during an emergency meeting on Sunday.

M23 insurgency has intensified, with the Rwanda-backed rebels seizing control of more territory than ever before, prompting the U.N. to warn of the risk of a broader war. South Africa said nine of its peacekeepers were killed in fighting.