The Ugandan government said 10,000 guerrillas have surrendered under a three-month-old amnesty and announced that it is extending the offer until December. The amnesty was to have ended today; officials declined to say how long it will last. President Yoweri Museveni announced the amnesty Aug. 5, when an estimated 50,000 rebels split among half a dozen groups were fighting his government. Museveni, whose rebel National Resistance Army seized power Jan. 25, 1986, promised pardons to all rebels who had not committed crimes against civilians. Reception centers have been established in 11 of the East African nation's 33 districts to feed and clothe surrendering rebels.