KABUL — The killing of five British troops by a rogue Afghan policeman underlines concerns about training and discipline within the ranks and possible insurgent infiltration of a police force that the U.S. hopes will be its ticket out of Afghanistan someday.

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 Barack 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, as were two Afghan policemen when the soldiers returned fire, officials said.

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, a main part of the U.S. strategy for the war.

"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."

In October 2008, a policeman threw a grenade and opened fire on a U.S. foot patrol, killing one soldier. The previous month, a policeman opened fire at a police station, killing a soldier and wounding three before he was fatally shot.