President Obama is still considering the US strategy in Afghanistan
A senior US diplomat in Afghanistan has become the first such person to resign in protest at the war against the Taliban, the Washington Post reported.
State department employee Matthew Hoh, posted to Zabul province, said he quit because he had doubts about why the US was fighting, the paper reported.
His resignation letter, written last month, prompted officials to offer him alternative jobs, but he declined.
President Barack Obama is considering future US strategy in Afghanistan.
In quotes published in the Washington Post, Mr Hoh wrote to the US state department personnel section: "I have lost understanding of and confidence in the strategic purposes of the United States' presence in Afghanistan.
"I have doubts and reservations about our current strategy and planned future strategy, but my resignation is based not upon how we are pursuing this war, but why and to what end."
White House 'ripples'
Mr Hoh said he decided to speak out because "I want people in Iowa, people in Arkansas, people in Arizona, to call their congressman and say, 'Listen, I don't think this is right'. "
The Washington Post described the resignation as sending "ripples all the way to the White House".
To prevent him leaving, officials offered Mr Hoh a job in the US embassy in Kabul. He was flown to Washington to meet the US special representative for Afghanistan and Pakistan, Richard Holbrooke, who initially persuaded him to remain to influence policy from within rather than outside government.
However, Mr Hoh ultimately refused, and his resignation became final in October.
He said that the insurgency in some parts of Afghanistan arrived only after the US troops did.
The White House has said a decision on a new strategy for Afghanistan, including troop numbers, will be made in the coming weeks, but critics say the process is taking too long.
The top US military commander in Afghanistan, Gen Stanley McChrystal, has asked for at least 40,000 more troops there.
Mr Hoh advised cutting combat troops; more support for Pakistan; better US communication and propaganda skills to match those of al-Qaeda; and more pressure on Afghan President Hamid Karzai to clean up government corruption, the Washington Post reported him as saying.
"We want to have some kind of governance there, and we have some obligation for it not to be a bloodbath," Mr Hoh said.
"But you have to draw the line somewhere, and say this is their problem to solve."