Barack Obama has begun what is arguably the most difficult task of his young presidency, launching his long-awaited new strategy for the war in Afghanistan and selling it to the public and U.S. allies.

The U.S. president to set to deliver a nationally broadcast speech Tuesday night, in which he will lay out why his country is in Afghanistan and how they will disengage from the conflict.

Obama has already issued orders for more troops for the campaign, believed to be in the 30,000 range, which would put more than 100,000 American soldiers in Afghanistan at a cost of about $75 billion a year.

Obama was to spend Monday personally talking to foreign leaders from France, Britain, China, Russia, India, Afghanistan and Pakistan.

Vice-President Joe Biden called Prime Minister Stephen Harper Monday evening to notify him of the U.S. plans, the Prime Minister's Office said.

"The prime minister and the president have had on several occasions, extensive conversations on Afghanistan, as recently as the most recent APEC Summit," spokesperson Dimitri Soudas said in an email.

Foreign Minister Lawrence Cannon told Â鶹ӰÊÓ Channel's Power Play he would be meeting with U.S. Secretary of State Hillary Clinton on Friday to discuss the Afghan mission.

Cannon said Clinton informed him of "general principles" of the American plan in a discussion last week.

Obama informed Clinton of his decision in a telephone conversation Sunday afternoon and then discussed the plan with other important administration players, such as Defense Secretary Robert Gates, in an Oval Office meeting in the evening.

The order for new deployments for the military became official at that time, White House Press Secretary Roberts Gibbs told reporters Monday.

He added that the goal of Obama's new strategy is to train Afghan forces to take over from the U.S., and that the president is not going to allow an open-ended commitment.

The president is not expected to announce a firm date for a withdrawal, but wants a gradual pullout over three to five years.

U.S. Army Gen. Stanley McChrystal wants an Afghan security force of about 400,000, along with 160,000 police officers, in place by October 2013.

Obama is also looking for more help from regional allies.

"Without partners willing to do stuff in both Afghanistan and Pakistan, no number of American troops can solve all of those problems," Gibbs said.

Britain confirmed on Monday it was sending 500 more troops to Afghanistan, putting 9,500 British soldiers in total in the country.

Canada has pledged to end its combat mission in Afghanistan in 2011.