Pakistan is closing its border with Afghanistan after allowing 3,000 refugees to enter.
It is the largest number of Afghans crossing into Pakistan since the US-led air campaign began.
Many are fleeing Kandahar, the Taliban headquarters.
The United Nations reports the city to be 80% empty after intensifying bombing in recent days.
"They arrive here penniless, knocking on everyone's doors, begging for food, money, anything," said Mr Ajnabi Gul Aga, resident of the Pakistan border town of Chaman.
The head of the UN relief agency, Mr Ruud Lubbers, claims hundreds of thousands of civilians are on the move.
Afghanistan's neighbours have closed their frontiers to the refugees, fearing a deluge of both civilians and armed fighters. Surrounding countries already host millions of Afghan refugees from two decades of conflict there.
After allowing in the biggest single-day refugee influx in the air campaign, Pakistan told UN officials it will keep its border closed.
PA