The US ambassador in Kabul has written to the White House arguing against sending thousands more American troops to Afghanistan, BBC reported.
In a leaked cable, Karl Eikenberry expressed doubts about the competence of President Hamid Karzai's government.
The message arrived amid intense debate over strategy, with President Obama yet to make a decision on troop numbers.
This dramatic intervention seems to put the envoy at odds with generals wanting reinforcements, correspondents say.
On Wednesday, Mr Obama he held his eighth meeting in a series aimed at refocusing Afghan policy.
Mr Eikenberry, a former US commander in Afghanistan, also raised concerns about corruption within the Afghan government.
He said it was "not a good idea" to send more troops, the BBC has been told.
The cable arrived days before Mr Obama held a crucial strategy session - to discuss the question of whether to send tens of thousands more troops to confront and push back the Taliban.
The cable appears to be a dramatic and last-minute intervention by the ambassador, BBC Washington correspondent Adam Brookes reports.
It comes right at the end of weeks of White House deliberation over how to proceed in Afghanistan, and appears to put the ambassador at odds with the US army, whose generals favour reinforcing and intensifying America's campaign in Afghanistan.
Military commander 'fuming'
The top US military commander in Afghanistan, Gen Stanley McChrystal, has asked for at least 40,000 more US troops.
The US currently has some 68,000 troops in Afghanistan, contributing to a coalition force of more than 100,000.
Gen McChrystal was "fuming" about Mr Eikenberry's intervention, sources said.
At Wednesday's meeting, White House officials said Mr Obama discussed timeframes for four options presented at the meeting but took no decisions on them.
The president said the US commitment should not be open-ended and governance there must improve, they added.
Among the key outstanding issues is said to be the reliability of the government of President Karzai, who was recently declared the winner of a widely criticised election.
Critics have said the decision on Afghanistan is taking too long, while Mr Obama has said he does not want to rush a decision that involves putting troops at risk.