Eight years after the Sept. 11 attacks led the U.S. into Afghanistan, Obama said it is still in America's vital national interest to "dismantle and destroy" al-Qaida terrorists and extremist allies. "I intend to finish the job," he said.
Military officials expect an infusion of approximately 32,000 to 35,000 troops to begin in February or March, the largest expansion since the beginning of the war and one that could bring the cost above $75 billion annually.
Obama said he would announce after Thanksgiving his decision on additional troops, and military, congressional and other sources said the occasion would be a Tuesday night televised speech laying out his plans for expanding the Afghan conflict - and then ultimately ending America's military role.
Republican critics have been pressing Obama for months to decide on a next step in Afghanistan, but Obama has said repeatedly he was more concerned with making a decision that was right rather than quick.
Neither he nor his advisers has detailed an exit plan, but the strategy he is expected to describe next week would include specific dates that deployments could be slowed or stopped if necessary, a senior military official said. The official and others spoke on condition of anonymity because the decision was not final.
With U.S. combat deaths climbing on Obama's watch and more than half the American public opposed to escalation, the president seemed to acknowledge Tuesday that he has a lot to explain.
"I feel very confident that when the American people hear a clear rationale for what we're doing there and how we intend to achieve our goals, that they will be supportive," he said, speaking at a White House news conference with Indian Prime Minister Manmohan Singh.
"I can tell you, as I've said before, that it is in our strategic interest, in our national security interest to make sure that al-Qaida and its extremist allies cannot operate effectively" in the area, he said. "We are going to dismantle and degrade their capabilities and ultimately dismantle and destroy their networks. And Afghanistan's stability is important to that process."
Returning to a campaign theme, Obama said the Afghan effort had been starved for resources and attention during the Bush administration and he intended to finish the war.
To that end, much of the White House discussion during months of deliberations has centered on how the U.S. would end its military role.
Obama held his 10th war council meeting Monday evening, and officials said it was his last. Military officials have said Obama is choosing one of the least risky options he was presented, but one still expected to lead to increased U.S. casualties without guarantee of success.
War commander Gen. Stanley McChrystal has warned that the war risks failure without a large troop infusion. Although he preferred a higher figure - about 40,000 - McChrystal is expected to tell Congress next week that this lesser addition still gives him the tools to better combat insurgents in the south and east of Afghanistan.
The expected increase would include at least three Army brigades and a single, larger Marine Corps contingent, officials said.
Both Democrats and Republicans in Congress has been miffed that the administration blocked McChrystal from testifying during what many Republicans considered an inordinately long decision-making period. His testimony has not been scheduled, but would probably come late next week or early in the week after.
Among others likely to take part in congressional hearings are Secretary of State Hillary Rodham Clinton, Defense Secretary Robert Gates, Joint Chiefs of Staff Chairman Adm. Mike Mullen and U.S. Ambassador to Afghanistan Karl Eikenberry. All were among about 20 officials and advisers participating in the president's final deliberations Monday night - one of the biggest groups gathered for these sessions.
The administration figures will have a tough sell among some congressional Democrats, including Senate Armed Services Committee Chairman Carl Levin, D-Mich., who has questioned the value of adding forces and pointed to the war's rising cost.
The Afghan war bill hit $43 billion annually this summer, with the addition of 21,000 forces Obama has already added to the fight this year.
AP White House Correspondent Jennifer Loven contributed to this report.