U.S. Army Gen. Stanley McChrystal met with President Barack Obama for 25 minutes on Friday. (Kirsty Wigglesworth/Associated Press)

U.S. President Barack Obama and his top Afghan war commander met privately aboard Air Force One Friday for a talk the White House described as productive.

The 25-minute meeting with Army Gen. Stanley McChrystal as the presidential jetliner waited to bring the president home from Denmark, gave Obama a chance to step outside the circle of advisers he has convened to study the problem of Afghanistan.

His war council has been sharing differing opinions on whether the United States should send thousands more troops to try to tamp down the Taliban insurgency or should shift to a narrower focus on al-Qaeda in neighbouring Pakistan.

The Copenhagen tarmac meeting was an extension of those war council sessions "as we reassess and re-evaluate moving forward in Afghanistan," White House press secretary Robert Gibbs told reporters afterward.

He said Obama and McChrystal "both agree that this is a helpful process." No decisions were made at their meeting, Gibbs said.

Obama was in the Danish capital to pitch Chicago's bid to host the 2016 Olympic games, and McChrystal was summoned there from London, where he gave a speech on Thursday warning that insurgents are gaining strength in Afghanistan and the United States and NATO need to increase their efforts there.

Hours after Obama and McChrystal met, the Pentagon said the general's official request for more troops for the war will not be sent to the White House until next week at the earliest.

At issue is Obama's looming decision to stick with the current mission in Afghanistan, which could require adding as many as 40,000 additional U.S. troops, or scale back the military option and expand operations targeting terrorists in Pakistan.

Pentagon spokesman Bryan Whitman said Friday that McChrystal's troop request still is at the Pentagon, and a decision is not imminent. "I don't see anything in the short term. It is exceptionally, closely held," he said.

Without citing numbers publicly, McChrystal has said more troops are needed to "buy time" for the Afghan military and police forces to prepare to take control of the country in 2013.

The meeting was the third conversation between the commander in chief and McChrystal since the general disclosed in a television interview that aired Sunday that he had spoken with Obama only once since he took over the U.S. military efforts in Afghanistan.

Obama tapped McChrystal in May to replace ousted Gen. David McKiernan. Obama and McChrystal spoke on Wednesday before Obama convened a meeting later that day of his war council, which McChrystal joined by video conference.

The president had long been expected to approve McChrystal's plan to mount a military push against the Taliban in Afghanistan.

It is possible, however, that Obama will decide on a hybrid strategy that keeps in place the 68,000 U.S. combat troops who are already in Afghanistan, while adding more military trainers and ramping up strikes on suspected al-Qaeda leaders in Pakistan.

Obama's strategy review was prompted in part by a critical assessment of the war effort that McChrystal sent him last month.

Military commanders support the current strategy of targeting the Taliban, but opinions were divided among other top officials.