The head of NATO and US Vice President Joe Biden joined forces Saturday to urge European allies to step up efforts in Afghanistan, as the United States prepares to send in thousands more troops. NATO Secretary General Jaap de Hoop Scheffer warned the Europeans that they were undermining their leadership credentials and upsetting the balance within the world's biggest military alliance, as it battles a Taliban-led insurgency. "I am frankly concerned when I hear the United States is planning a major commitment for Afghanistan, but other allies ruling out doing more," he said, at a major international security conference in Munich, southern Germany. "That is not good for the political balance of this mission. That is not good for the balance inside the North Atlantic alliance," he said. "Leadership and burdens -- they go together." Scheffer, who did not single out any nation, warned that the failure to step up "makes calls for Europe's voice to be heard in Washington perhaps a bit more hollow than they should be."