Soviet troops are delaying their pullout from Kandahar, Afghanistan's second largest city, Western diplomats said today. Kandahar, population about 200,000, has been under siege for several weeks and Moslem guerrillas are reportedly preparing for another major offensive against Soviet-Afghan troops stationed there. Reports reaching Islamabad say Afghan insurgents have pushed government troops to defensive positions around the outskirts of the city. The Soviet Union in 1979 sent troops to Afghanistan to support the Marxist regime. On May 15, the Soviets began a withdrawal, which has been slowed by attacks from guerrillas. Diplomats, speaking on condition of anonymity, said a Soviet regiment was told to cancel plans to leave Kandahar, reflecting Soviet fears over the city's vulnerability. Soviet forces reportedly have left Ghazni, on the road from Kandahar to the Afghan capital of Kabul, but some soldiers remain in Jalalabad, where the Soviet withdrawal was launched in May. There have been reports of heavy fighting in the northeastern province of Baghland. The Salang highway, stretching 200 miles north of Kabul to the Soviet border, is becoming increasingly dangerous, sources said. It was closed to civilian traffic for five days ending Sunday to allow departing Soviet troops to move north. Diplomatic sources say the convoys have been attacked by guerrillas and that up to 100 Soviet soldiers were reported killed in recent clashes. The official Kabul radio, monitored in Islamabad, continues to broadcast a list of guerrilla defeats and atrocities. It claimed Monday that 93 guerrillas were killed and as many injured in recent clashes in seven of Afghanistan's 28 provinces. The various reports cannot be independently confirmed.