Robert Gates said writing cheques gave donors leverage over corruption
International donors must do more to ensure their aid does not encourage corruption in Afghanistan, US Defence Secretary Robert Gates has said.
Mr Gates said donors should tighten control of how the "significant influx" of money into the country is used.
He was speaking a day after President Hamid Karzai was sworn in for a second term after a controversial election.
Western leaders have put pressure on Mr Karzai to deal with corruption and remove former warlords from government.
US President Barack Obama - who is considering whether to send up to 40,000 more troops to tackle the Taliban in Afghanistan - has also spoken out about the need to fight corruption in that country.
Mr Gates said the presence of international forces and contractors in Afghanistan had "provided a significant influx of assistance dollars and contracts".
He said the US and other foreign donors should act whenever they found signs of corruption in their contracts or development projects with Afghan or international groups.
"The place to start is the place where we have the greatest leverage and that's where we're writing the cheques," he said. Mr Gates' comments, at a news conference in Halifax, Canada, echoed remarks by US Secretary of State Hillary Clinton on Thursday.
She said the US was working to ensure US funds in Afghanistan were "accounted for and used as intended".
The BBC's Paul Adams in Washington says that at a time when the US public is increasingly sceptical about the war and its cost, the Obama administration is clearly seeking to give reassurance that the cause is worthwhile.
Mr Karzai was sworn in as president on Thursday, following an election which was tainted by widespread fraud.
He was declared president after a second round election run-off was abandoned when his sole remaining challenger, Abdullah Abdullah, pulled out, saying the vote could not be free and fair.
In his inauguration speech, Mr Karzai pledged to clean up the country's politics, saying corruption was "a dangerous problem".
Mr Karzai said he would organise a conference in Kabul to "organise new and effective ways to combat this problem".
Earlier on Friday, at least 16 people were killed and several wounded by a bomb in the south-west of the country.
Police said a suicide bomber in a motorbike detonated the device in a crowded market in Farah city, capital of Farah province.
Also on Friday, a controversial former Afghan warlord narrowly escaped an assassination attempt which killed at least five of his bodyguards in Paghman district, north of Kabul.
It is not clear who attacked Abdul Rasul Sayyaf, who is an ally of Mr Karzai and now an MP.