On 16/17 September the air fleets were notified of the Soviet invasion of Poland. "Luftflotte 1" support operations against troop concentrations ceased in central and southern Poland to avoid friendly-fire incidents. Kesselring attempted to crush Polish resistance by making a series of air attacks against Warsaw in the final week of September. With the military campaign virtually over, Polish resistance was confined to the Hel Peninsula, Warsaw, and Modlin. Kesselring's air fleet was assigned to the north of the city. In the ensuing attacks, approximately 10 per cent of the city's buildings were destroyed and 40 per cent damaged. The bombing killed between 20,000 and 25,000 civilians. Kesselring insisted that only military targets were attacked, but the lack of precision munitions made the bombing indiscriminate and militarily ineffective.