The Obama administration secretly shipped $400 million in cash to Iran around the time that the country released four American prisoners, The Wall Street Journal reported on Tuesday.

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The money — which consisted mainly of euros, Swiss francs and other foreign currencies — was airlifted to Iran in an unmarked cargo plane in January.

The payment was part of a settlement over an arms deal from 1979, shortly before Iran went through a regime change.

U.S. officials have denied that the payment was a ransom for the release of the prisoners.

“As we’ve made clear, the negotiations over the settlement of an outstanding claim ... were completely separate from the discussions about returning our American citizens home,” said John Kirby, a State Department spokesman.

“Not only were the two negotiations separate, they were conducted by different teams on each side, including, in the case of The Hague claims, by technical experts involved in these negotiations for many years.”

Still, Republican lawmakers, all of whom opposed the Iran nuclear agreement that was passed last year, have criticized President Obama for making payments to Iran.

Sen. Tom Cotton Tom Bryant CottonBattle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates Chris Wallace presses Cotton on 'any hypocrisy' between comments on Supreme Court vacancy in 2016 and today MORE (R-Ark.) said the transaction amounted to “a $1.7 billion ransom to the ayatollahs for U.S. hostages.”

“This break with longstanding U.S. policy, put a price on the head of Americans and has led Iran to continue its illegal seizures” of Americans, he added.