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 extremists and injured four, according to Pakistani intelligence officials.

A campaign of U.S. drone strikes against extremists in the tribal areas has dramatically accelerated this year, targeting members of groups including the Taliban, an offshoot known as the Haqqani network, and al-Qaeda.

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 extremists 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.

The first of the strikes took place at midmorning in the Mir Ali district of the North Waziristan tribal area, the officials said. Two trucks were hit as they drove along a dirt track after leaving a compound in the village of Sher Tala, a known insurgent stronghold.

Masked extremists cordoned off the scene, preventing anyone from getting close while they carried away the dead and injured, villagers said.

Hours later, a separate strike targeted a vehicle in the village of Machikhel, also in Mir Ali. That attack killed four people, according to officials.

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, which occurred near a major bank branch and a police compound, and more than a dozen were injured.

In the late summer and early fall, NATO forces managed to clear Taliban fighters from several key districts surrounding Kandahar, part of a push to restore government authority in the main city in southern Afghanistan. But Western officials acknowledged that the gains were fragile and could be reversed if insurgents poured back into the area in the spring.

Also Monday, the Western military reported the death in southern Afghanistan of a service member, whose nationality was not immediately disclosed. This year has been the most lethal of the nine-year war for U.S. troops and the NATO force as a whole.

At least 496 Americans have died this year in Afghanistan and 1,443 since the war began, according to the icasualties.org website.

Americans make up about two-thirds of the 150,000 foreign troops in Afghanistan.