Pakistan said 26 prisoners were freed late last year in a bid to kick-start peace talks ahead of the withdrawal of US-led Nato troops from Afghanistan, whose government is under pressure from an 11-year Taliban insurgency.
"We don't know, frankly, what has happened to the people that the Pakistanis have released," ambassador James Cunningham told a news briefing.
"We would have preferred to have greater visibility into that, but still it's positive that they were released, I think, from the Afghan point of view."
With the control of prisoners in Afghanistan a major issue between the US and the government of President Hamid Karzai, Cunningham said some freed prisoners had returned to Taliban ranks in senior positions in the past.
A peace process "hasn't even really begun", he said.
"Our goal is the beginning, if not the conclusion of, a serious process on peace and reconciliation as soon as possible – but so far it hasn't proven possible... to get that going," he said.
Washington began tentative moves towards peace with the Taliban a year ago. But the Islamists broke off the talks a few months later, apparently over the failure of the US to free Taliban prisoners held at Guantanamo Bay.
Now the US is negotiating its exit from its longest war, pledging continued support for the Afghan government against an undefeated insurgency.
But details of US involvement after Nato combat troops withdraw in 2014 remain vague and subject to a security agreement still under negotiation.
It had been expected that a residual force of up to 20,000 troops would remain behind to help counter the Taliban, who were ousted in 2001 for harbouring al Qaeda leader Osama bin laden.
Now US officials have suggested as few as 3,000 troops may remain behind, raising questions about what effect they could have in a country which many predict will plunge back into civil war.
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wow.. what a joke...