Pakistani military officials say the country's security forces have killed at least 40 militants in a tribal area bordering Afghanistan over the past several days.

Officials said Thursday that Pakistani forces went on the offensive three days ago after suffering repeated attacks by militants in the volatile Mohmand tribal agency.

The area is largely closed to journalists and outsiders because of ongoing violence, so the number of casualties could not be independently confirmed.

Mohmand is one of seven federally administered tribal areas lying in the mountainous border zone between Pakistan and Afghanistan. The tribal areas are mainly ethnic Pashtun and are considered a stronghold for Taliban and al-Qaida militants.

The Pakistani military has spread its operations against militants throughout several of the tribal areas, while U.S. unmanned aircraft continue to fire missiles at suspected militant outposts.

While Washington does not publicly discuss its missile strikes in Pakistan, the issue remains a sore spot with Islamabad.

Pakistan publicly complains that the missile strikes infringe on its sovereignty. However, regional experts say Pakistan assists the United States behind the scenes by identifying targets.

Pakistan is still dealing with the fallout from the covert U.S. helicopter raid deep into its territory that killed al-Qaida leader Osama bin Laden in early May. The Pakistani military was unaware of the raid until it was occurring in the garrison town of Abbottabad near the capital Islamabad.

Pakistani officials complained that the United States planned for and launched the raid without their knowledge, showing a lack of trust in a country that Washington cites as one of its biggest allies in the war on terror.