GOMA, Democratic Republic of Congo (Reuters) -Rwandan-backed rebels marched into the centre of east Congo's largest city on Monday, witnesses said, in the latest escalation of decades of conflict in a region already suffering one of the world's worst humanitarian crises.

Fighting with M23 rebels in eastern Democratic Republic of Congo left six United Nations peacekeepers dead, UN officials said Saturday.

M23 rebels in the Democratic Republic of Congo were reported to have taken the key eastern city of Goma in fierce fighting with government forces that sent streams of refugees fleeing area communities.

In a statement, the rebels urged residents of Goma to remain calm and for members of the Congolese military to assemble at the central stadium.

The M23 militia, funded and directed by Rwanda, said it had seized the city of Goma, terrifying its people, many of whom sought shelter there after fleeing the rebel advance.

Advances being made by the M23 rebel group in the Democratic Republic of Congo are "heightening the threat of a regional war," according to U.N. Secretary General Antonio Guterres.

Britain, the EU, and the United Nations have piled pressure on the African nation over its backing of the M23 rebels

M23 rebels said they have advanced into Goma, a large city in the eastern Democratic Republic of Congo. Hundreds of thousands of residents have been displaced.

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.