COPENHAGEN - U.S. President Barack Obama met the man heading U.S. and NATO troops in Afghanistan, General Stanley McChrystal, Friday, a White House spokesman said.

The meeting on board Air Force One in Copenhagen lasted 25 minutes, spokesman Robert Gibbs told reporters on the plane.

McChrystal flew to Denmark from London, where he met with Prime Minister Gordon Brown Thursday, and delivered a stark assessment of the Afghanistan insurgency to military and defense experts at the International Institute of Strategic Studies.

Gibbs declined to give details of the discussion between the president and McChrystal, which followed a high-level meeting on Wednesday in Washington between Obama and his top foreign policy advisers over the deteriorating situation in Afghanistan.

As the casualty toll grows and popular support for the campaign wanes, Obama is carrying out a major strategy review of his Afghanistan policy, which could either lead to more troops being sent or a cutback in the 68,000-strong U.S. force there.

Obama was in Copenhagen to urge the International Olympic Committee to choose Chicago as host city of the 2016 Olympics.

(Reporting by Jeff Mason; Writing by Elizabeth Fullerton; Editing by Michael Roddy)