Pakistani security forces say they have been exchanging heavy fire with Taliban militants on the outskirts of the main town in the northwestern Swat valley.
Authorities have called on civilians who are still inside the town of Mingora to leave if they can.
The military on May 17 said "intense" exchanges of fire had broken out as forces advanced on Mingora, where many Taliban fighters are believed hiding out. Forces have also entered the towns of Matta and Kanju to battle militants.
Interior Minister Rehman Malik claimed that more than 1,000 militants have been killed since the military offensive began three weeks ago.
The toll could not be independently confirmed, and no information was available on civilian casualties.
The United Nations says the offensive has forced more than 1 million people in the region from their homes, on top of more than 500,000 people who were displaced by fighting in the northwest last year.
compiled from agency reports