Kerry's stance would aim for a modest increase in American forces, treading middle ground between Republicans who have said Obama would put soldiers and the country at risk by rejecting McChrystal's larger request and anti-war Democrats who question whether the United States has already taken on too much in Afghanistan.
"Under the right circumstances, if we can be confident that military efforts can be sustained and built upon, then I would support the president should he decide to send some additional troops to regain the initiative," Kerry said.
Obama is nearing a decision on whether to commit large numbers of additional troops to the war next year. McChrystal favors an increase of roughly 40,000, officials have told The Associated Press, which would allow the U.S. military to expand its reach in areas of the country's south and east now under Taliban sway.
Fresh from several days of talks in both Pakistan and Afghanistan, Kerry warned that the United States also cannot risk a drastic shift in strategy that would focus narrowly on hunting terrorists.
"We all see the appeal of a limited counterterrorism mission, and no doubt it is part of the endgame, but I don't think we're there yet," Kerry said during remarks to the Council on Foreign Relations. "A narrow mission that cedes half the country to the Taliban could lead to civil war" in Afghanistan and threaten the fragile civilian government in Pakistan, he said.
As Obama weighs his options on the military side, the State Department said Monday it was on track to meet the goal of tripling the size of the civilian component in Afghanistan by year's end or very early 2010.
That will bring the number of agronomists, lawyers, diplomats and development experts in the country from 320 in January to 974, Deputy Secretary of State for Management Jack Lew told reporters.
Lew said he did not expect Obama's decision on troops to have a significant effect on the civilians except in cases where additional troops might secure new areas of the country for them to work safely.
Three civilian Drug Enforcement Administration agents died Monday during the crash of a U.S. military helicopter that also killed seven U.S. service members, an official said. The craft went down in the west of Afghanistan.
The casualties mark the first DEA deaths in Afghanistan since the drug agency began operations there in 2005.
The official spoke on condition of anonymity because no formal announcement has been made. Officials say the helicopter had left the scene of a fire fight with insurgents.
Associated Press writers Matthew Lee and Devlin Barrett contributed to this report.