U.N. observers encountered what they believed to be about 100 Rwandan troops in eastern Congo, a U.N. official said Wednesday, marking the first U.N. sighting since Rwanda threatened to send in its forces against Rwandan Hutu rebels sheltering there.

The suspected Rwandan forces, seen near the largely unmarked border area of Rutshuru, withdrew toward Rwanda after Tuesday's encounter, said M'hand Ladjouzi, head of the U.N. mission at Goma (search). He spoke at a news conference in Goma, the largest city of the east.

Rwanda President Paul Kagame (search) on Tuesday told lawmakers in his country that Rwanda would act against 8,000 to 10,000 Rwandan Hutu (search) rebels sheltering in east Congo, saying that a five-month-old U.N.-led disarmament program had failed to neutralize the Rwandan Hutu rebel forces.

U.N. officials have said previously that extensive sweeps by their more than 11,000-strong force in Congo had failed to turn up any signs of Rwandan incursions since Rwanda's threat arose.

But Ladjouzi said that "these kinds of infiltrations are not new" and that a joint patrol with Congolese troops last week arrested nine Rwandan troops who remain in Congolese custody. But he provided no further details.

Ladjouzi said U.N. forces were investigating reports of three villages being burned between the towns of Rutshuru and Lubero.

As tensions rise, large numbers of Rwandan Hutu rebels have begun moving west out of the Rutshuru region, sending civilians in the area fleeing, he said. Those refugee flights were not caused by Rwandan forces, he said.

"If Rwandan forces target the civilian population, MONUC will take action," he said, using the U.N. acronym for its mission in Congo.

Rwanda has twice invaded eastern Congo, in 1996 and 1998, to hunt down Rwandan Hutu combatants responsible for the 1994 genocide of more than a half-million minority Tutsis (search) and moderate Hutus.

The 1998 invasion sparked a five-year war that drew in the armies of four other African nations and split Western Europe-sized Congo. An estimated 3.2 million people died, most through famine and disease.

Peace accords by 2002 saw the withdrawal of foreign armies and establishment of a power-sharing government.

Small scale incursions of Rwandan forces have been alleged since then, but not confirmed.