A rebel militia known as M23 has seized the key Congolese city of Goma, threatening displaced civilians and raising fears of a broader regional war.

Some 400,000 people have fled their homes in eastern Congo as the rebels advanced on Goma, according to the U.N. refugee agency. In the last few days, many more have arrived in the city.

M23 is funded and directed by the government of Rwanda, according to the United Nations and the United States. Rwanda denies any direct involvement with the group.

The conflict has its roots in Rwanda’s 1994 genocide, which spilled over the border into Congo, leading to decades of fighting. Like the leaders of Rwanda, M23 is mostly made up of people from the Tutsi ethnic group.