Intelligence sources in Pakistan say Afghanistan is bracing for retaliation from the United States following Tuesday's terrorist attacks on U-S targets.
Muslim militants are reported to be fleeing the capital Kabul, while other residents are said to be digging trenches around the city.
The Associated Press quotes Pakistani security sources as saying prime suspect Osama bin Laden moved to a new hiding place in Afghanistan shortly after Tuesday's attacks.
The exiled Saudi militant is known to run training camps in eastern areas of Afghanistan, where mountain caves offer many hideouts.
Afghanistan's Taleban rulers have repeatedly denied that bin Laden was behind Tuesday's attacks.
In 1998, the United States launched cruise missiles against sites in Afghanistan where bin Laden was suspected of training terrorists. The U-S strikes came after deadly bomb attacks on two U-S embassies in Africa.
Meanwhile, Pakistan intelligence sources quoted by the Washington Post say the Taleban's radical leader Mohammad Omar has left his headquarters in the southern Afghan city of Kandahar and is also in hiding.
Pakistan is one of only three countries that recognizes Afghanistan's Taleban government.