Pakistani fighter jets killed 13 suspected militants in fresh airstrikes on Tuesday, the military said.

War planes targeted seven suspected hideouts used by militants in the North Waziristan tribal district near the Afghan border, an Army statement said, as the offensive continued into a fourth week.

Defence Minister Khawaja Asif said on Monday that more than 400 militants had so far been killed since the operation was launched on June 15.

Troops fighting to retake control of a key town held by rebels for several years were facing “stiff resistance,” a military official said.

Ground troops backed by air power and artillery were engaged in “fierce” gun battles around Miranshah, considered the headquarters of militants linked with al-Qaeda, the official said.

Mr. Asif said 20 soldiers had been killed in more than three weeks of the campaign.

Civilian authorities in nearby Bannu town were struggling to cope with a huge influx of people fleeing North Waziristan.

An official at the National Disaster Management Authority said more than 800,000 displaced people had been registered, prompting fears that there might be shortages of food and medicine.