Washington-based Special Inspector-General for Afghan Reconstruction said in a report earlier this year that the country may not be ready for peace unless it finds a way to reintegrate Taliban fighters into society and combat "endemic corruption".
The United States ambassador to Afghanistan on Thursday openly questioned the Afghan government's commitment to fighting corruption after reports that a key figure in a massive banking scandal received an early prison release in return for a large campaign donation.
Ambassador John Bass's comments came after local media reports alleged that former Kabul Bank chief executive Khalilullah Ferozi was released into house arrest after a $30 million donation to President Ashraf Ghani's election campaign.
Presidential spokesman Sediq Seddiqi said that those reports are "a big lie" and that Ferozi was transferred into house arrest because of his deteriorating health.
The US comments further strain relations with the Afghan government as a US envoy and the Taliban near a deal to end America's longest war.
The talks have excluded Ghani's government, as the Taliban describe it as a US puppet.
"To be honest, I'm very sad to see this," Seddiqi said of the ambassador's comments posted on Twitter.
The Kabul Bank scandal in 2010 drew global headlines and became a test of the Afghan government's ability and willingness to address widespread corruption.
The country's largest private bank nearly collapsed, and Ferozi and chairman Sherkhan Farnood were sent to prison and told to repay more than $800 million.
Farnood recently died in prison, and Ghani's spokesman said the president approved Ferozi's transfer into house arrest after the judiciary system looked into his health.
The move was a "humanitarian act" and one to help ensure that Ferozi would stay alive to repay the hundreds of millions of dollars, Seddiqi said.
He dismissed the idea of a campaign donation by Ferozi, saying that "if he had this much money, he would have" repaid it as required.
Ferozi has less than a year left in his sentence, Seddiqi said.
The US ambassador said he was "disturbed" by allegations first raised this week by presidential candidate and former national intelligence chief Rahmatullah Nabil.
"Countless Afghans suffered in the past decade because international assistance funds were stolen for personal gain," the ambassador said.
Washington-based Special Inspector-General for Afghan Reconstruction (SIGAR), which monitors billions of dollars in US aid to the country, said in a report earlier this year that Afghanistan may not be ready for peace unless it finds a way to reintegrate Taliban fighters into society and combat "endemic corruption."