U-S military aircraft and personnel are in Pakistan in connection with the U-S-led military campaign against terrorists in Afghanistan.
Pakistani officials say Islamabad is allowing U-S forces to use two airfields in the southern part of the country, but not to launch attacks.
The officials acknowledge that U-S personnel had already arrived in the country, but stress that Pakistan will not be used as a base for offensive action against Afghanistan. They added that the assistance is part of President Pervez Musharraf's agreement to provide logistical support for any operation.
At least 15 U-S military aircraft, including C-130 transport planes, are reported to have landed at Pakistan's Jacobabad air base in southern Sindh province.
Pakistan is the only country that maintains diplomatic relations with Afghanistan's ruling Taleban.
Islamabad has ruled out allowing Pakistani territory to be used to stage attacks on Afghanistan because of the possibility of a backlash from sections of its own population.
Clashes this week between police and demonstrators in violent protests in several Pakistani towns left at least five people dead.