Muslim guerrillas have stepped up fighting near Afghanistan's capital, Western diplomats said. The insurgents rocketed Soviet hilltop positions in Paghman, just west of Kabul, on Nov. 2 and 4 and exchanged fire with a Soviet base on Friday, they said. The diplomats, based in Islamabad, Pakistan, said Kabul is under tight security. Afghan government forces have even stopped diplomatic vehicles at gunpoint and have threatened to fire on the cars of two Western embassy chiefs, they said. In New York, the U.N. General Assembly, for the ninth year, called on the Soviet Union to withdraw its troops from Afghanistan.