Mrs Clinton said the US was reviewing its approach to militancy
US Secretary of State Hillary Clinton has said whoever wins Afghanistan's recent election will be expected to do more to address the country's problems.
Speaking to the BBC while in London, Mrs Clinton said the next leader needed to build better relationships with the US, the army and the Afghan people.
She said America's goal in Afghanistan was still to defeat al-Qaeda.
But the current US review of the conflict was "leading to some welcome clarity" on the best tactics, she said.
Mrs Clinton, currently on a European tour, told the BBC's Today programme that the US was "anxiously awaiting" the outcome of the presidential elections which were held in Afghanistan in August.
The results have been delayed over accusations of fraud and malpractice.
Preliminary results indicate that the incumbent, Hamid Karzai, leads with about 55% of the vote - considerably ahead of his nearest rival Abdullah Abdullah, who has 28%.
BBC diplomatic correspondent James Robbins said Mrs Clinton appeared unusually hesitant when asked whether the US would be proud to stand beside Mr Karzai if he emerged as the winner.
She said simply that the president had been "very helpful on many fronts".
"We often overlook the progress made in Afghanistan because of the serious challenges that still exist," she said.
"But we are very clear that, if this election results in him being re-elected, there must be a new relationship between him and the people of Afghanistan, between his government and governments which are supporting the efforts in Afghanistan to stabilise and secure the country."
Mrs Clinton said the next president would also have to do more to train and deploy Afghan forces to take over from foreign troops.
"It is a more complex picture than sometimes emerges from snapshot views. But clearly we expect more; we're going to be working towards more," she said.
Late on Sunday, the New York Times quoted senior administration officials as saying the US president was impatient and "not satisfied" with progress on developing civil institutions, the judiciary and security forces in Afghanistan.
President Barack Obama's civilian goals had been largely unmet, the officials said.
Mr Obama announced in March he would deploy hundreds of civilians to work in the country but officials told the paper that, because of deteriorating security, many aid workers could not travel outside the capital to advise farmers.
The US president is currently undertaking a review of the US military involvement in Afghanistan and the wider region, eight years after the operation began.
The commander of US forces in Afghanistan, Gen Stanley McCrystal, has formally requested a significant increase in troop numbers.
Mr Obama is reported to have ruled out troop cuts or a major scaling back of the US effort in Afghanistan, but it remains unclear whether he will approve a significant escalation of an increasingly unpopular war.
Mrs Clinton described the review process as "a very thorough scrubbing" of US strategy which was "leading to some welcome clarity".
She said America's aim in the region was still "to achieve the goal of disrupting, dismantling and defeating al-Qaeda and its extremist allies", but that it was now adopting "a much more careful analysis of who actually is allied with al-Qaeda".
"We want to be smart about how we are proceeding. The lives that our young men and women, both American and British, have put at risk - and lost - are very much in our minds. We intend to get this as right as is humanly possible."
Mrs Clinton said many people had been paid or coerced to fight with the extremists - and developing partnerships with such people would yield results, as had been the case in Iraq.
The secretary of state's visit comes at a time of increasing volatility in the region.
At the weekend, some 19 people died when militants ambushed an army base in Rawalpindi in neighbouring Pakistan, an attack blamed by Pakistan on the Taliban and al-Qaeda.
Mrs Clinton said the attack on Rawalpindi, an army town to the south of the capital, Islamabad, showed militants in Pakistan were increasingly threatening the authority of the state.
But she said the US saw no evidence they were going to succeed, or that Pakistan's nuclear arsenal was under threat.