Egon Ramms told German public radio station Deutschlandfunk that the situation would only worsen later this year when US forces withdraw some of their troops from Afghanistan if NATO did not send more troops.
"We are talking about a total of 5,000, 6,000 soldiers," he said. "But we need these soldiers now and very soon as we need to hold onto certain areas because we need to create trust among the Afghan population."
Ramms said the alliance wanted to hand over that responsibility to the Afghan forces when they are prepared by at the latest 2012.
"In other words, the forces that I am now lacking could delay the withdrawal of NATO and ISAF," the NATO-led International Security Assistance Force," he said.
Ramms said NATO also needed to speed up the training of Afghan police forces, adding that it had fallen two-and-a-half years behind the training of the Afghan army. He also called for more contact between ISAF and Afghan civilians, saying it would undermine the power of insurgents.
"The key to success in fighting the Taliban is with the local population," he said, adding that in regions where ISAF enjoyed the trust of residents, it was better informed about planned attacks.
Despite the presence of about 70,000 international troops from some 40 countries mainly operating under NATO, an insurgency aimed at toppling the US-backed government in Kabul has gained pace in the past two years. German Defence Minister Franz Josef Jung said Saturday he favours bolstering Germany's military presence in Afghanistan - currently fixed at 3,500 troops - by another 1,000 soldiers.
Parliament is to decide later this year on prolonging Germany's participation in the NATO force and reshaping its mission amid repeated NATO calls for it to bolster its presence in the country.