AFP reports hundreds of people leaving Marjah, a city of 80,000 in Helmand province.
A large force of U.S. Marines and Afghan soldiers is expected to launch an assault on Taliban forces there within days.
"The government of Afghanistan will reclaim Marjah as one of its own," said the British commander of the operation, General Nick Carter.
AFP reports NATO forces have been dropping leaflets in the region for weeks, warning residents of the impending assault.
A Taliban spokesman told AFP the insurgents were massing fighters around Marjah and "ready to fight."
Analysts say the assault -- known as Operation Mushtarak, Dari for "together" -- will be the first big test of U.S. President Barack Obama's deployment of 30,000 extra U.S. troops to Afghanistan.
compiled from agency reports
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