German Gen. Egon Ramms, whose Allied Joint Forces Command oversees NATO's force in Afghanistan, also said it was impossible to guarantee that incidents such as this week's killing of five British soldiers by an Afghan policeman would not happen again.

"The clock in Afghanistan is clearly running against us," Ramms told reporters at a news conference held in an underground command center known as Castlegate.

Ramms noted some nations _ primarily Canada and the Netherlands _ already have announced plans to start drawing down their forces operating in Afghanistan as part of the 71,000-strong NATO security force. Other nations also are looking into withdrawing some of their troops.

These moves come as Stanley McChrystal, the NATO and U.S. commander in Afghanistan, is calling for tens of thousands more U.S. and allied troops to bolster the international forces combating Taliban militants.

Public opinion polls show that that with the Afghan war dragging on into its ninth year, many Europeans oppose sending more soldiers at a time of economic crisis and shrinking defense budgets.

"What I fear most is that we will lose the support of the Afghan people," Ramms said. "But I also fear the impatience of the populations in our own Western countries. In the end, politicians will not go against their public opinion."

Ramms addressed the growing doubts over whether NATO and U.S. troops could trust the Afghan soldiers and policemen they were training, following the deaths on Tuesday of the five British soldiers. They were gunned down by an Afghan policeman.

It was unclear whether the attacker, who escaped after the shooting, was just a disgruntled officer or a member of the Taliban.

"You cannot be sure that somebody is not working for the other side," Ramms said.

He also said that the U.S. pilots who bombed a pair of hijacked fuel tankers in a German-ordered air strike on Sept. 4 near the northern town of Kunduz, killing dozens of Afghans, had repeatedly sought clarification from ground controllers before they dropped their bombs.

"They asked the ground station whether to destroy the tankers or target the people gathered around them," Ramms said. "They then asked to do a demonstration of force to scare off the people gathered there before dropping the bombs on the tankers."

The accidental killing of civilians by NATO forces has infuriated Afghans and has deepened skepticism in the West over the war _ particularly in Germany, where the Kunduz airstrike has become a major political issue.

NATO has prepared a classified report on the incident which has been submitted to German political leaders.