The U.N. World Food Program says almost half a million earthquake survivors in Pakistan have yet to receive relief supplies.
The agency's director James Morris says the relief effort is one of the most challenging the world has ever faced.
The UN warning came as Pakistan's President Pervez Musharraf appealed to the international community for more tents to shelter the homeless.
Skies were sunny again on Tuesday, and Pakistani and U.S. military helicopters delivered aid at a brisk pace to Muzaffarabad, which suffered most of the damage and casualties from last week's massive earthquake. Relief workers rushed to set up field hospitals to treat thousands of injured people. Officials say the death toll is already close to 50,000 in Pakistan alone.
Military officials say big choppers are bringing aid to forward bases in the Neelum and Jhelum valleys and then smaller helicopters take it to remote mountain villages.
The United Nations says more than 80 helicopters were flying, and that the world body was planning to send up to 150,000 tents for the homeless, in addition to about 30,000 already distributed.
India had also offered helicopters. But Monday it rejected Pakistan's suggestion that it would take the helicopters, but without Indian pilots.