Secretary-General Anders Fogh Rasmussen said some critics are starting to say that the cost of engagement in the eight-year 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 at a security conference in Bratislava ahead of a NATO defense ministers meeting that opened late Thursday with a closed-door dinner.

He said the allies must do more to enable Afghan forces to eventually assume responsibility for security in their country.

NATO currently has 59 training teams working with the Afghan army. Alliance officials say they need the allies to come up with nine more to fulfill present plans that call for an expansion of the Afghan forces from the present 94,000 to 134,000. But if a future expansion plan boosting the Afghan army to 400,000 troops is approved, NATO will need a total of 103 training teams on the ground.

The NATO chief is pushing for greater cooperation between NATO and Russia. He said he hopes to persuade 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 shipments of supplies to coalition forces in Afghanistan across its territory, but has ruled out sending troops. The Soviet Union lost 15,000 soldiers in its war in Afghanistan in the 1980s before it was forced to retreat in humiliation.

It was in the chaos after the Soviet withdrawal that the Taliban came to power in 1996.

Fogh Rasmussen's warning about the dangers of disengaging from Afghanistan were echoed Thursday by Prime Minister Robert Fico of Slovakia, the meeting's host. Fico said "we cannot allow Afghanistan to once again become a haven for terrorism" and pledged to beef up Slovakia's 250-member engineering unit based in Kandahar and Uruzgan provinces.

The top U.S. and NATO commander in Afghanistan will make an unexpected appearance Friday at the meeting. Officials said late Thursday that Army Gen. Stanley McChrystal would brief NATO officials in a series of meetings on his ground-level view of the war in Afghanistan.

McChrystal has called for more troops, and Fogh Rasmussen hopes NATO members can endorse that recommendation.

The Bratislava meeting also comes as U.S. Vice President Joe Biden visits Central Europe, with stops in Poland, Romania and the Czech Republic. He is discussing Obama's new U.S. missile defense plans and reassuring the strongly pro-American region of Washington's continued support, 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, which would include U.S. Navy ships equipped with anti-missile weapons _ such as the Navy's Standard Missile-3 _ forming a front line of defense in the eastern Mediterranean. Those would be combined with land-based anti-missile systems to be placed on land in Europe.

Though Slovakia has not been asked to take part, Fico, the Slovak leader, said he would adamantly refuse if ever approached.

"I will not allow the deployment of any component of the missile system in Slovakia," Fico told reporters during a news conference at which he also condemned the U.S. invasion of Iraq in 2003 as being "only motivated by oil."

Fogh Rasmussen also plans to propose to the 28 ministers a sweeping reform of NATO's military structure which would enable more of the of the alliance's 2.5 million service members to be used in operations, a spokesman said. Currently, less than half of those forces are deployable, and only about 10 percent are sustainable on missions for any length of time.

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Associated Press writer Lara Jakes contributed to this report from Bratislava.