ISLAMABAD, Pakistan, (AP) – Pakistani forces seeking to conquer a Taliban and al-Qaeda stronghold near the border with Afghanistan killed 15 insurgents in ongoing clashes, officials said Monday.

There was no word of casualties on the government side. It was not possible to verify the toll in the Bajur region, where journalists cannot move freely because of poor security.

U.S. officials have praised the operation in Bajur, which Pakistan’s army says has been serving as a sanctuary for insurgents fighting on both sides of the frontier. However, it is only one of several militant hotspots in Pakistan’s border region and the army says its troops have run into stiff and well-organized resistance.

Fazl Rabi, a local police official, said troops repelled an overnight attack by 50 militants against a camp about 6 miles north of Khar, Bajur’s main town. He said militants also attacked paramilitary troops before dawn in the Tang Khata area of Bajur.

Rabi and another government official counted a total of 15 militants killed and more than a dozen wounded in overnight clashes.

Two intelligence officials said three troops died in fighting in the past two days. They asked for anonymity because they were not authorized to talk on the record to media.

Meanwhile, artillery fire directed at suspected militant hideouts in an area called Badali struck two houses and killed two civilians, said Niazur Rehman, a local resident.

Bajur is the most northerly of Pakistan’s wild tribal regions, several of which have fallen largely under the control of militants opposed to the Afghan and Pakistani governments. The army claims to have killed more than 1,000 militants in the two-month-old offensive in Bajur and lost more than 60 troops. It has declined to estimate casualties among civilians.

The United Nations announced Monday that 20,000 Pakistani refugees have fled to Afghanistan to avoid fighting between militants and Pakistan’s military.

The U.N.’s refugee agency said nearly 4,000 Pakistani families have fled Bajur into Afghanistan’s Kunar province.

The border zone is considered a likely hiding place for al-Qaeda leader Osama bin Laden.

Pakistani President Asif Ali Zardari said in remarks aired on Sunday that he had no idea where bin Laden was. He also reiterated his appeal to U.S. authorities to halt unilateral raids on militant targets in Pakistani territory, insisting Pakistani forces could be trusted to do the job.