BRATISLAVA, Slovakia -- 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 Qaeda 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 Qaeda. 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.

The NATO chief is pushing for greater cooperation between NATO and Russia. He said he hopes to convince Moscow to become more engaged in Afghanistan, where a resurgent Taliban has been making gains and forcing a U.S. review of its strategy there.

Russia has given some support to the international anti-terrorism campaign in Afghanistan, allowing transports over its territory, for instance. But it has not sent troops, and there are mixed signals on how fully it backs the operation. Moscow itself got bogged down in a devastating war there in the 1980s that left 15,000 Soviet soldiers dead and proved a humiliating defeat for the Soviet superpower.

Fogh Rasmussen's warning about the dangers of disengaging from Afghanistan were echoed Thursday by French Defense Minister Herve Morin, who said on RTL radio that "if the Taliban take power tomorrow, terrorism can prosper and one day or another will strike all western democracies."

Morin said the press is painting a darker picture of Afghanistan than reality suggests.

"In reality, today there is a national Afghan army that is becoming a real army of more than 60,000 men, which progressively is taking in hand the situation and military operations," Morin said. Taliban attacks can occur even in improving areas, "giving the impression that things are going from bad to worse."

U.S. Defense Secretary Robert Gates is expected to brief allies in Bratislava about progress in a review of recommendations by American Gen. Stanley McChrystal, the top U.S. and NATO commander in Afghanistan. McChrystal has called for more troops, and Fogh Rasmussen hopes NATO members can endorse that recommendation.

The Bratislava meeting comes as U.S. Vice President Joe Biden visits Central Europe, with stops in Poland, Romania and the Czech Republic. He is discussing the new U.S. missile defense plan and reassuring the strongly pro-American countries that Washington still supports them, even though it scrapped George W. Bush's plans for missile defense sites in Poland and the Czech Republic.

Russia strongly opposed Bush's plan and has welcomed President Barack Obama's shift away from it.

On Wednesday, Biden secured Poland's willingness to participate in Obama's reconfigured missile defense system for Europe.

Jaroslav Baska, the Slovak defense minister and the host of conference, said Wednesday he expects Gates to also explain changes in U.S. missile defense policy and how it would affect NATO.

"Any kind of defense that can secure Europe is welcome. If this becomes part of NATO's own missile defense then it is doubly welcome," he said.

Fogh Rasmussen said he wants Russia brought into NATO's missile defense plans. He said it appears more likely with Obama's plans to reconfigure the Bush-era missile defense project.