The Rwanda-backed M23 armed group was threatening another key town in eastern Democratic Republic of Congo on Friday, as the United Nations top rights body said it would launch an investigation into alleged violations and abuses during the deadly clashes gripping the African nation.

U.N. human rights chief Volker Turk said on Friday that the worst could be ahead in the escalating crisis in eastern Congo, saying he expected sexual violence including incidents of rape and sexual slavery to increase.

The U.N. secretary-general appealed Thursday for peace in the Democratic Republic of Congo, where a recent escalation of fighting between rebels and the national army has killed at least 2,900 people and displaced tens of thousands.