The British prime minister is in the Afghan capital, Kabul, for talks with Afghan President Hamid Karzai.
Gordon Brown arrived in Afghanistan Saturday on a previously unannounced visit. He met with British troops in southern Helmand province before traveling to the capital.
British military officials say it is the closest the British prime minister has ever gotten to the front lines in Afghanistan.
His visit comes just one day after four British soldiers were killed in Helmand - one by a roadside bomb and three by a 13-year-old suicide bomber.
Mr. Brown denounced the Taliban for using such tactics.
The prime minister released a statement calling the soldiers' deaths a "tragic loss" and saying fighting the Taliban puts British forces in the front line against terrorism.
In a separate development, U.S. and Afghan forces say they killed six suspected militants and detained 16 others in operations in southern Afghanistan Thursday and Friday.
Elsewhere in the country's south, NATO said its troops fired on a speeding bus that failed to heed warning shots on Friday, killing at least three civilians on board.
The alliance says a NATO foot patrol fired the warning shots as the bus veered toward the troops on the highway that links Kabul to Kandahar.
Civilian deaths at the hands of international troops have caused tension between the Afghan government and Western governments.
Mr. Karzai has repeatedly called on international forces to do all they can to prevent civilian casualties during military operations.