The plan may have political advantages, allowing Obama to approach the start of his expected 2012 reelection campaign by arguing that the military mission in Afghanistan is on the downslope, even though few troops will have returned home by then.
M. Ashraf Haidari, political counselor at the Afghan Embassy in Washington, praised the plan for its signs of commitment, saying the administration will not withdraw troops if they are needed in the fight.
Haidari said he does not expect the United States to "prematurely disengage from Afghanistan . . . that would be a recipe for disaster."
Obama is seeking wider international support for the stepped-up mission that he called in his speech "a test of the common security of the world."
In Brussels, NATO Secretary-General Anders Fogh Rasmussen said Wednesday that allies in the war effort "will send at least 5,000 more soldiers and probably more" to Afghanistan next year.
"This is not a U.S. mission alone. America's allies in NATO have shared the risks, costs and burdens of this mission from the beginning," said Rasmussen, who welcomed Obama's announcement of a troop increase as "proof of his resolve."
French President Nicolas Sarkozy called the Obama speech courageous, but said a decision to send more troops to Afghanistan required further review.
In Germany, officials said they were prepared to conduct more police training in Afghanistan, but would not determine whether to send more troops until a strategy review early next year.
In the United States, several members of Congress expressed particular concern at the possibility that the Obama plan would result in greater militancy in the region.
"We cannot support your decision to prolong and expand a risky and unsustainable strategy in the region," wrote Sen. Russell D. Feingold (D-Wis.) and two House members in a letter to Obama. "There is a serious danger that the ongoing, large-scale U.S. military presence will continue to provoke greater militancy in the region and further destabilize both Afghanistan and nuclear-armed Pakistan."
Times staff writer Henry Chu in Copenhagen contributed to this report.