Republican lawmakers are fuming over a bombshell report overnight that the U.S. government airlifted the equivalent of $400 million to Iran this past January – as four detained Americans were released by Tehran – and say the transaction has put more Americans at risk of being taken hostage.

The cash transfer, as reported by The Wall Street Journal, was the first installment paid in a $1.7 billion settlement the Obama administration reached with Iran to resolve a failed 1979 arms deal dating from just before the Iranian Revolution.

The cash flown to Iran consisted of euros, Swiss francs, and other currencies because U.S. law forbids transacting American dollars with Iran. While the Obama administration denied the cash transfer was done to secure the release of the four Americans, GOP lawmakers said it was tantamount to “ransom.”

"Paying ransom to kidnappers puts Americans even more at risk," Sen. Mark Kirk, R-Ill., said in a statement. "While Americans were relieved by Iran’s overdue release of illegally imprisoned American hostages, the White House’s policy of appeasement has led Iran to illegally seize more American hostages."

The four released Americans are Washington Post reporter Jason Rezaian; Amir Hekmati, a former U.S. Marine; Christian pastor Saeed Abedini; and Nosratollah Khosravi-Roodsari, whose case had not been publicized before the release.

State Department spokesman John Kirby said their release was on an entirely separate track from the settlement payment and, in an interview Wednesday with Fox News, said any suggestion it was tied to ransom is “utterly false.”

“We just don’t pay ransom. … This was not ransom,” he said.

Kirby further said there was “no secret” about the payment.

“It was their money,” Kirby told Fox News, noting the funds had been frozen and it “made no sense for us to continue to drag out their claim.” He argued the “compromise” struck with Iran worked to the “taxpayers’ benefit” because potentially billions more could have been on the line.

In a written statement, Kirby said the negotiations over the Americans and over the settlement were even conducted "by different teams on each side." He said: "The funds that were transferred to Iran were related solely to the settlement of a long-standing claim at the U.S.-Iran Claims Tribunal at The Hague."

However, the Journal says U.S. officials acknowledge that Iranian negotiators on the prisoner exchange said they wanted the cash to show they had gained something tangible.

The Journal also reported that President Obama did not disclose the $400 million cash payment when he announced Jan. 17 that the arms deal dispute had been resolved. The administration has not disclosed how the $1.7 billion was paid, except to say it was not paid in dollars.

“The logistics of this payment -- literally delivering a plane full of cash to evade U.S. law -- shows yet again the extraordinary lengths the Obama administration will go to accommodate Iran, all while hiding the facts from Congress and the American people,” House Foreign Affairs Committee Chairman Ed Royce, R-Calif., said in a statement. “Hundreds of millions in the pockets of a terrorist regime means a more dangerous region, period. And paying ransom only puts more American lives in jeopardy.”

Since the cash was airlifted, Iran's Revolutionary Guard has arrested two more Iranian-Americans. Tehran also has detained dual-nationals from France, Canada and the U.K. in recent months.

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