GOMA, Congo (AP) – 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 reported U.N. sightings since Rwanda threatened to send in its forces against Rwanda Hutu rebels sheltering here.

The suspected Rwandan forces withdrew toward Rwanda after Tuesday’s encounter, said M’hand Ladjouzi, head of the U.N. mission at Goma. He spoke at a news conference in Goma, the largest city of the east.

A Rwandan diplomat denied Rwanda had invaded again, after a week of warnings that raised fears of a return to the six-nation war that devastated Congo, Africa’s third-largest nation.

But the denial came even as a Western envoy in Kinshasa, Congo’s capital, said Rwandan President Paul Kagame warned that Rwandan troops would carry out “surgical strikes” against rebels in a letter to U.N. Secretary-General Kofi Annan.

In the letter, which circulated among embassies in Congo on Wednesday, Kagame said military operations would last two weeks, according to the envoy, who spoke on condition of anonymity.

U.N. officials in Kinshasa said they had no knowledge of the letter. Congo, meanwhile, asked the U.N. Security Council to meet in emergency session to condemn Rwanda’s threat and impose sanctions finding Kagame “personally responsible for the threat posted to the sovereignty of Congo and to the entire peace process in the region.”

Kagame told Rwandan lawmakers Tuesday that Rwanda would act against 8,000-10,000 Rwanda Hutu rebels based in east Congo, saying a five-month-old U.N.-led disarmament program had failed to neutralize the Rwandan Hutu rebel forces.

In Kinshasa, Congo’s capital, U.N. spokeswoman Patricia Tome said Wednesday that Rwanda’s threat “astonished” the U.N. mission in Congo, as it came at a time when authorities hoped to speed up the U.N.-led disarmament effort.

U.S. State Department spokesman Adam Ereli urged Rwanda and Congo “to solve their differences diplomatically and not militarily, through the exchange of gunfire or the movement of troops in the area.”

Until Wednesday, U.N. officials said extensive sweeps by their more than 11,000-strong force in Congo had turned up no signs of Rwandan incursions since Rwanda’s threat.