Aid organizations fear a new humanitarian crisis in the restive eastern Congo region, where the renowned armed rebel group M23 is in the midst of a new advance that threatens to cut off a major city and leave millions of people struggling for food and medical help.

Eastern Congo has been beset by conflict for years, with M23 among more than 100 armed groups vying for a foothold in the mineral-rich area near the border with Rwanda. Some have been accused of carrying out mass killings.

There’s been an upsurge in fighting in recent weeks between M23 rebels and Congo army forces, and it comes as the United Nations plans to withdraw peacekeepers from the region by the end of the year.

Tensions are also rising between Congo and Rwanda, with them blaming each other for supporting various armed groups. Congo accuses Rwanda of backing M23.

the U.S. State Department condemned what it called the “worsening violence.” A group of aid agencies has estimated that 1 million people have already been displaced by fighting in the last three months.

WHO ARE M23?

The March 23 Movement, or M23, is a rebel military group mainly made up of ethnic Tutsis that broke away from the Congolese army just over a decade ago. They staged a large offensive in 2012 and took over the provincial capital of Goma near the border with Rwanda, the same city they are threatening again.

The conflict has regional complications, with neighboring Rwanda also accused by the U.S. and U.N. experts of giving military aid to M23. Rwanda denies that but effectively admitted on Monday that it has troops and missile systems in eastern Congo. Rwanda said that is to safeguard its own security because of what it claims is a buildup of Congo army forces near the border. Rwanda has rejected calls from the U.S. to withdraw.