Afghanistan is the most complex challenge that NATO has ever undertaken, but the alliance must remain engaged there to prevent the country turning back into an al-Qaida training ground, the organization's top official said Thursday. Secretary-General Anders Fogh Rasmussen said some critics are starting to say that the cost of engagement in the 8-year-old war is too high, but he countered that "the cost of inaction would be far higher." "Leaving Afghanistan behind would once again turn the country into a training ground for al-Qaida. The pressure on nuclear-armed Pakistan would be tremendous. Instability would spread throughout central Asia and it would only be a matter of time until we here in Europe would feel the consequences of all of this," Fogh Rasmussen said in a speech at a security conference in Bratislava ahead of a meeting of NATO defense ministers.