The Obama administration has said that a 400 million dollar (£303 million) cash payment to Iran seven months ago was contingent on the release of a group of American prisoners.
It is the first time the US has so clearly linked the two events, which critics have painted as a hostage-ransom arrangement.
State Department spokesman John Kirby repeated the administration’s line that negotiations to return the Iranian money – from a military-equipment deal with the US-backed shah in the 1970s – were conducted separately from talks to free four US citizens in Iran.
But he said the US withheld the delivery of the cash as leverage until Iran permitted the Americans to leave the country.
“We had concerns that Iran may renege on the prisoner release,” Mr Kirby said, citing delays and mutual mistrust between countries that severed diplomatic relations 36 years ago.
He said that as the US “of course sought to retain maximum leverage until after the American citizens were released. – that was our top priority”.
Both events occurred on January 17, fuelling suspicions from Republican politicians accusations from party presidential nominee Donald Trump of a deal that undermined America’s long-standing opposition to ransom payments.
In a speech in Charlotte, North Carolina, Mr Trump accused President Barack Obama of lying.
“He denied it was for the hostages, but it was. He said we don’t pay ransom, but he did. He lied about the hostages, openly and blatantly,” Mr Trump said.
Mr Kirby spoke a day after The Wall Street Journal reported new details of what happened on January 17.
US officials would not let Iran bring the cash home from a Geneva airport until a Swiss Air Force plane carrying three of the freed Americans departed from Tehran, the paper reported. The fourth American left on a commercial flight.
Earlier this month, after the revelation the US delivered the money in pallets of cash, the administration flatly denied any connection between the payment and the prisoners.
“Reports of link between prisoner release & payment to Iran are completely false,” Mr Kirby tweeted at the time.
The money comes from an account used by the Iranian government to buy American military equipment in the days of the shah.
The equipment was never delivered after the shah’s government was overthrown in 1979 and revolutionaries took American hostages at the US Embassy in Tehran. The two sides have argued over the account and numerous other financial claims ever since.
Mr Obama has said his negotiators secured the US a good deal on a busy diplomatic weekend that also included finalising the seven-nation nuclear accord. But he and other officials have consistently denied any linkages.
“We actually had diplomatic negotiations and conversations with Iran for the first time in several decades,” Mr Obama said on August 5, meaning “our ability to clear accounts on a number of different issues at the same time converged”.
He stressed: “This wasn’t some nefarious deal.”
Some Iranian officials immediately linked the payment to the release of four Americans, including Washington Post reporter Jason Rezaian, who had been held in Iranian prisons.
Another of the prisoners, pastor Saeed Abedini, also linked the two events.
He said as the prisoners waited for hours at an airport to leave Iran, a senior Iranian intelligence official informed them their departure depended on the plane with the cash.