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The NATO secretary-general called for greater engagement in Afghanistan, saying the war there is the most complex challenge the alliance has ever undertaken.

Speaking ahead of a meeting of NATO defense ministers in Slovakia Thursday, Anders Fogh Rasmussen said the alliance must remain committed to bringing stability to Afghanistan. 

Noting the reservations some NATO members have about heightening engagement in Afghanistan, Rasmussen said "the cost of inaction would be far higher."

U.S. Secretary of Defense Robert Gates is expected to brief the conference on America's review of its war strategy.

U.S. President Barack Obama is considering a new strategy in Afghanistan, including a request from the top U.S. general in the country, Stanley McChrystal, for tens of thousands more troops.

NATO ministers meeting for two days of talks in Bratislava also are expected to discuss the United States' revamped plans for a missile defense system for Europe.

Some information for this report was provided by AP and Reuters.