PESHAWAR, Pakistan- Missiles presumably fired by U.S. drones on Monday incinerated three trucks thought to be ferrying fighters and weapons from Pakistan’s tribal borderlands to Afghanistan. The strikes killed 25 suspected militants and injured four others, according to Pakistani intelligence officials.

A campaign of American drone strikes against militants in the tribal areas has dramatically accelerated this year, targeting members of groups including the Taliban and al-Qaida.

Insurgent sanctuaries inside Pakistan are considered a tremendous complicating factor for Western forces in Afghanistan. NATO ground troops are not allowed to operate in Pakistan, although a few incursions have taken place. Drones have become a key means of hunting militants who attack Western soldiers, then take shelter on the Pakistani side of the frontier.

But the drone strikes are politically unpopular in Pakistan, and the Pakistani government routinely denies having given U.S. forces the green light to carry them out.

Violence also broke out on the Afghanistan side of the border. An explosion tore through a busy intersection in the southern city of Kandahar.

At least two people were killed in the blast.