Pakistan is reportedly sending troops to its border with Afghanistan.
The move comes as Pakistan closed its border with its neighbour.
With the exception of food, nothing else is getting through the border, including hundreds of Afghans trying to cross into Pakistan.
The closure of the border is one of several requests made of Pakistan by the US.
UN refugee officials say tens of thousands of Afghans have left their cities and are heading for the borders with Pakistan and Iran.
The United Nations High Commission for Refugees says people are trying to leave because they fear US reprisal attacks.
Most people are leaving Kabul, the southern city of Kandahar and eastern Jalalabad, near the border with Pakistan. Others are heading toward Iran.
Pakistan and Iran have both sealed their borders to stop the influx of refugees. Millions of displaced Afghans are already in the two countries.
Aid agencies have pulled all their workers out of Kabul after the Taliban ordered them to leave, saying it could not guarantee their safety in the event of a US assault.
The International Red Cross, which rarely withdraws from war-torn nations, has pulled out its last 15 foreign aid workers.