KALAM, Pakistan — U.S. Army choppers flew their first relief missions in Pakistan's flood-ravaged northwest today, airlifting hundreds of stranded people to safety from a devastated tourist town and distributing emergency aid.

Elsewhere, authorities began evacuating half-a-million people as Pakistan's worst monsoon rains in decades threatened new destruction.

The floods have already killed an estimated 1,500 people over the past week, most in the northwest, the center of Pakistan's fight against al-Qaida and the Taliban. An estimated 4.2 million Pakistanis have been affected, including many in eastern Punjab province, which has seen numerous villages swallowed by rising water in recent days.

The flooding is one of several crises that has hit Pakistan since mid-July, including a suicide bombing in the northwest city of Peshawar, a plane crash that killed 152 people in the capital, and a spurt of politically motivated killings that have left dozens dead in the southern city of Karachi.

Four U.S. Chinook helicopters landed in the resort town of Kalam in the Swat Valley, which has been cut off for more than a week, according to an Associated Press reporter there. They flew hundreds of people — many of them vacationing there — to safety lower down the valley, he said.

A U.S. Embassy spokesman said 800 people were evacuated and relief goods distributed.