The Soviet Union says it has had to fly thousands of tons of food supplies into the besieged Afghan capital of Kabul and blamed the pro-Moscow regime of President Najibullah for the shortages. Soviet television said Sunday that U.S.-backed Moslem rebels have tightened their grip on Kabul, where basic foodstuffs such as rice and wheat flour have nearly run out. "Kabul is besieged . . . by groups of bandits from the opposition," said veteran Soviet television reporter Mikhail Leshinsky from the Afghan capital.