The former NATO commander in Afghanistan said Friday the United States may need to keep a larger force in the country than initially planned after 2014 to help Afghans build up their military.

His suggestion, endorsed by a former senior official at the Pentagon, would expand the potential US military commitment to Afghanistan beyond next year and appeared to contradict President Barack Obama's stated policy on the war.

John Allen, the retired four-star general who oversaw the NATO-led war effort for more than a year and half, had previously recommended keeping 13,600 troops in Afghanistan beyond the deadline before he stepped down from his post in February.

The Obama administration is currently negotiating a long-term security agreement with the Afghan government and has yet to announce how many troops it hopes to keep in Afghanistan after 2014, when the bulk of NATO combat forces are scheduled to withdraw.

But in a report made public that Allen co-authored, the former commander endorsed establishing a temporary "bridging force" for up to three years -- over and above whatever troop levels are declared as America's long-term presence after next year.

"Finally, for two to three years after 2014, the United States may need an additional force package of several thousand personnel to help the Afghans finish building their air force, their special operations forces and certain other enablers in medical realms, in counter-IED (improvised explosive devices) capability and in intelligence collection," the report said.

Along with Allen, the report was written by the former number-three ranking civilian at the Pentagon who shaped Afghan policy, Michele Flournoy, and an analyst at the Brookings Institution, Michael O'Hanlon.

"The bridging force for a couple of years could be additional to what you define as your enduring force," O'Hanlon told reporters.

If Afghan troops needed temporary help with air power, medical evacuations or other capabilities, this force could fill in "gaps" while the Afghans got up to speed, he said.

O'Hanlon added that "in theory, you could have a larger force than you anticipated for a short time."

Allen said the idea was not new to the military and that there already was a bridging force in Afghanistan in charge of closing down bases, but not counted as part of the overall troop presence.

"It's a force that's not on the books, that does nothing everyday but close down bases," the ex-commander said.

Allen's comments came as Obama said that NATO will hold a summit next year to look at the "final chapter" in its Afghanistan war and what will be required after 2014.

Allen, Flournoy and O'Hanlon discussed their report on the war effort at the Center for a New American Security, a Washington think tank with close ties to the Obama administration.

The report also urged the White House to announce its decision on troop levels beyond next year "as soon as possible" to allay concerns among Afghans who fear the United States will pull out of the country entirely.

"Clarifying the US commitment would be very reassuring to the Afghans, it would address their historical fears of abandonment," Flournoy said.

"It would also help persuade our NATO allies to stay in the game and do their part."

Flournoy, the former undersecretary of defense for policy who resigned from her post last year, said she expected the administration to unveil its troop plans soon.

"From what I hear, that is likely to happen very soon."