"We cannot afford to lose what we have already achieved in the country," Jaap de Hoop Scheffer told a joint news conference with Portuguese Prime Minister Jose Manuel Durao Barroso. NATO can contribute to stop Afghanistan "from being a safe haven for terrorists" again, he said.
NATO took charge of the 6,100-troop International Security Assistance Force (ISAF) in Afghanistan last year which is mostly concentrated in Kabul.
The United Nations has authorized NATO to expand its peacekeeping mission beyond the Afghan capital, but because of a lack of resources the alliance has so far only managed to send several hundred troops to the quiet northern city of Kunduz.
But since taking office last month, de Hoop Scheffer has toured European capitals to persuade NATO nations to send more troops to Afghanistan.
Asked if there is willingness on the part of the 19 NATO members to contribute more forces to ISAF, he said: "There definitely is. It is a matter of translating political commitments into resources."
The NATO chief said there was a need for more soldiers to provide security in the war-ravaged country as well as to take part in provincial reconstruction teams --groups of up to 400 soldiers and civilian experts who carry out small developments projects crucial to rebuilding the country.
ISAF was formed shortly after US forces in 2001 ousted the hardline Taliban regime, which harboured the extremist Al-Qaeda network, from power.
Members of the Taliban militia continue to stage regular attacks on coalition and Afghan government troops, as well as aid workers, in a bid to undermine the government of President Hamid Karzai which replaced them.