The Obama administration admitted Thursday that a $400 million cash payment to Iran in January was contingent on the release of American prisoners being held in the country – while still denying that the payment was a ransom.

State Department spokesman John Kirby said that the negotiations to return the money – originally from a 1979 failed military equipment deal made between Iran and the U.S. – were conducted separately from negotiations to free the four prisoners.

The four detainees who were released on January 17 were Washington Post reporter Jason Rezaian; former U.S. Marine Amir Hekmati; Christian pastor Saeed Abedini and Nosratollah Khosravi-Roodsari, whose case had not been publicized before the release.

However, Kirby said that the U.S. withheld the cash delivery until Iran made good on its promise to release the prisoners.

“In basic English you are saying you wouldn’t give [Iran] the 400 million in cash until the prisoners were released, correct?” asked a reporter at Thursday’s State Department briefing.

“That’s correct,” Kirby responded.

The new details, first reported by The Wall Street Journal, added to criticisms from Republicans that it was a ransom paid by the Obama administration. Kirby’s admission only served to add fuel to the controversy.

“What the State Department admitted today was the dictionary definition of a ransom payment and a complete contradiction of what they were saying just two weeks ago,” RNC Spokesman Michael Short said in a statement. “It’s time for the Obama White House to drop the charade and admit it paid a ransom to the world’s leading state sponsor of terrorism.”

"If it quacks like a duck, it's a duck. If a cash payment is contingent on a hostage release, it's a ransom,” said Sen. Ben Sasse, R-Neb. “The truth matters and the President owes the American people an explanation."

Earlier this month, after the revelation the U.S. 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," Kirby tweeted at the time.

"This wasn’t some nefarious deal," Obama said during an August 4 press conference. "We do not pay ransom for hostages."

The agreement was the return of the $400 million, plus an additional $1.3 billion in interest, terms that Obama described as favorable compared to what might have been expected from a tribunal set up in The Hague to rule on pending deals between the two countries.

Abedini has claimed that he and the other hostages were kept waiting at an Iranian airport for more than 20 hours before their departure. Abedini said he was told by a senior Iranian intelligence official that their departure was contingent on the movement of a second plane.

State Department officials denied Abedini's claims to the Journal, saying the delay was due to issues locating Rezaian's wife and mother, who accompanied him on the flight.

According to the Journal, GOP leaders say they plan to hold hearings on the payment next month, when Congress returns from its summer recess. Rep. Sean Duffy, R-Wis., chair of the House Subcommittee on Oversight and Investigations, last week sent letters to the Justice and Treasury Departments, as well as the Federal Reserve, requesting more information the transaction.

The Associated Press contributed to this report.