Despite repeated air attacks by UN aircraft on the advancing North Korean troops, by early September the UN armies had been squeezed down into a small area south of the Naktong River, called the "Pusan Perimeter", and there was a very real fear that the hard-pressed UN troops might be forced to evacuate the entire Korean peninsula. In late July, MacArthur agreed to divert the B-29s to interdiction raids against North Korean targets nearer the 38th parallel in an attempt to interrupt supplies being delivered to North Korean troops in the south.