The attack caused anguish in Britain, where public support for the war has been waning. Britain is the largest contributor to NATO forces in Afghanistan after the United States, and its continued presence here is central to President Obama's strategy as he weighs dispatching tens of thousands more U.S. troops.

The five British soldiers, who had been advising Afghan policemen, were shot and killed Tuesday at a checkpoint where they were living in the volatile southern province of Helmand. Another six soldiers were wounded.

The gunman escaped and his motive was unclear.

The incident, which echoed two police shootings of U.S. soldiers last year, raised questions about whether international forces are trying to recruit and train Afghan police too quickly.

"There isn't a lot of vetting of police before they are hired," Peter Galbraith, the former top American official at the U.N. mission in Afghanistan, told BBC Radio 4.

In September, Sen. Carl Levin, D-Mich., called for increasing the size of the Afghan army and police "much faster than presently planned" instead of sending tens of thousands more Americans to fight here.

In Washington, Defense Department press secretary Geoff Morrell defended Afghan forces and the international training effort.

"However tragic and criminal this act was, it represents a rare and, luckily, thus far isolated incident. (NATO) troops continue to partner effectively with the Afghan national security forces and continue to build their capacity to take the lead in ultimately defending their country on their own."

Training and operating jointly with Afghan police and soldiers, as the British were doing Tuesday, are key to NATO's strategy of dealing with the spreading Taliban-led insurgency and, ultimately, allowing international forces to leave Afghanistan.

But obstacles are far greater with the police than with the army.

A Defense Department Inspector General report, released in September, found that Afghan police are crippled by serious corruption and subject citizens to frequent "shakedowns."

"Unlike the Afghan National Army, which is the most respected institution in the Afghan government, there is a wide consensus that many elements of the Afghan National Police are too corrupt, and too tied to politics and power brokers," wrote former Pentagon analyst Anthony Cordesman.

Downplaying the incident, Humayun Hamidzada, a spokesman for President Hamid Karzai, called it an isolated attack.

"In the U.S., people shoot up people in a shopping mall," Hamidzada said. "There are crazy people everywhere."

However, Karzai's main challenger in the recent election, former foreign minister Abdullah Abdullah, said the ongoing violence showed Karzai's administration has failed to stabilize the country despite eight years of assistance from international forces.

"In the absence of a credible and reliable and legitimate partner, more soldiers, more resources" are needed to fight the war, Abdullah told reporters.