Rwanda has vowed to attack a Rwandan rebel group in the eastern Democratic Republic of Congo, the United Nations said on Wednesday, warning that any such move would be a serious threat to stability in Congo. "Last night Rwanda announced that it was going to attack the FDLR (Democratic Forces for the Liberation of Rwanda) in Congo," said Patricia Tome, spokeswoman for the U.N. peacekeeping mission in Congo. "The United Nations cannot accept this kind of threat and the reaction of the international community will be very firm." In Kigali, a Rwandan official said rebels were advancing on its borders from bases in Congo and Kigali would use any means necessary to repel them. U.N. officials said they had spotted no sign of any major movement of fighters so far. The FDLR are largely drawn from the former army and Hutu militiamen that took part in Rwanda's 1994 genocide, killing some 800,000 Tutsis and moderate Hutus. Their numbers have been swelled by Hutu refugees living in eastern Congo since then. Rwanda has twice invaded Congo in the past decade to attack the rebels. The last assault in 1998 was one of the triggers for a five-year war in Congo that sucked in five other neighbouring states and killed three million, mostly from hunger and disease. --More 2059 Local Time 1759 GMT