Afghan police said heavy fighting had broken out between the Taliban and a force of German, Afghan and other foreign troops in Baghlan province. Provincial police spokesman Habib Rahman said three Afghan policemen were killed.
It was unclear if that fighting was related to the four NATO deaths.
Also Thursday, the international Red Cross reported that civilian injuries caused by roadside bombs and other explosives in southern Afghanistan have soared so far this year.
The Red Cross said the Mirwais Hospital it supports in the city of Kandahar admitted up to 40 percent more patients wounded by bombs in the first two months of the year compared with the same period last year, the group said in a report issued Wednesday. It said the wounded came from the surrounding province of Kandahar as well as neighboring Helmand, both areas in which the Taliban have a significant presence and have repeatedly engaged Afghan and foreign forces.
It said the Mirwais Hospital treated 51 patients for injuries caused by homemade bombs in March alone, well above the average monthly figure.
"Homemade bombs and improvised mines continue to pose a major threat to civilians in the south of Afghanistan. In the last few weeks, ICRC personnel at Mirwais Hospital in Kandahar have observed a substantial increase in casualties," the report said.
The insurgency employs roadside bombs and concealed homemade explosives to attack Afghan government forces and NATO troops and spread fear among the populace.
On Thursday, a scrap metal dealer and four children helping him were killed by a suspected homemade bomb as they loaded materials collected door-to-door in rural Kandahar onto a vehicle, provincial government spokesman Zulmai Ayubi said. Eight children were also wounded in the blast, which occurred near the town of Takhta Pul.
Elsewhere, one civilian was killed and one injured when a bomb blew up beneath the tractor they were riding near Gardez in the eastern province of Paktika, according to the Afghan Interior Ministry.
At least 2,412 Afghan civilians were killed in fighting last year, an increase of 14 percent from 2008, according to the United Nations. About two-thirds of the civilian deaths were a result of actions initiated by the insurgents, including ambushes, assassinations and roadside bombs, while the percentage of civilian deaths attributed to NATO and Afghan government forces fell.
Despite that, civilian deaths at the hands of U.S. and other international forces are a major concern because they're believed to fuel resentment of the Afghan central government in Kabul and generate sympathy for the insurgency.
On Thursday, the chief U.N. representative in Afghanistan called for greater attention to civilian safety in the wake of the deaths of four Afghans who died Monday when the bus they were traveling in was fired on by a U.S. military convoy. U.S. forces said they regretted the incident, which remains under investigation.
Staffan de Mistura called on "all parties to the conflict to do their utmost to minimize harm to ordinary Afghans and to take every possible precautionary measure to distinguish between civilians and combatants in their operations."
In his statement, de Mistura also said he appreciated new guidelines issued by the top U.S. and NATO commander in Afghanistan, Gen. Stanley McChrystal, that aim to limit use of force and minimize potential civilian losses.
Kandahar and Helmand are among a number of provinces that have seen a spike in violence over recent months as the Taliban seeks to reassert itself amid a surge in foreign troop levels.
Combined Afghan and international patrols on Wednesday uncovered a weapons cache in Kandahar's Arghandab district consisting of 108 rockets and about 200 feet (60 meters) of wire used to fire the weapons, NATO reported. Such weapons can be used to make roadside bombs or be fired at NATO bases.
NATO said another cache in Uruzgan province just north of Kandahar contained five anti-tank mines, 38 rocket-propelled grenades, 20 grenade launchers and six boxes of ammunition for the AK-47 rifle favored by insurgents.