The Obama administration said on Wednesday that $400 million in cash paid to Iran soon after the release of five Americans detained by Tehran was not ransom for them as some Republicans have charged.

The five, including Washington Post reporter Jason Rezaian, were released on Jan. 16 in exchange for seven Iranians held in the United States for sanctions violations. The prisoner deal coincided with the lifting of international sanctions against Tehran.

At the time, the United States said it had settled a longstanding Iranian claim at the Iran-U.S. Claims Tribunal in The Hague, releasing $400 million in funds frozen since 1981, plus $1.3 billion in interest that was owed to Iran.

The funds were part of a trust fund Iran used before its 1979 Islamic Revolution to buy U.S. military equipment that was tied up for decades in litigation at the tribunal.

"The link between prisoner release and payment to Iran are completely false," State Department spokesman John Kirby said on Twitter in response to a Wall Street Journal article that Washington secretly organized the cash airlift.

White House spokesman Josh Earnest heatedly beat back suggestions the money transfer to Iran was ransom, or a secret.

"The United States, under President Obama, has not paid a ransom to secure the release of Americans unjustly detained in Iran and we're not going to pay a ransom," he said at a daily White House briefing.

Earnest said the Republicans who have long opposed the Iran nuclear deal are seizing on how the money was paid to Iran as a way to undermine the deal. "They're struggling to justify their opposition to our engagement with Iran," he said.

"I understand the interest in details for a more colourful story but I don't understand what this does to the broader outlines of an agreement that has been in place for six months now."

While there have long been questions about the timing of the payment to Tehran, one Iranian concern was that the Obama administration would face too much domestic political criticism if it delayed acting on the tribunal's decision.

Due to the international sanctions against Iran, the payment, made in euros, Swiss francs and other currencies, had to be made in cash.

Republican presidential nominee Donald Trump blamed his opponent, former Secretary of State Hillary Clinton, for launching the talks with Iran.

"Our incompetent Secretary of State, Hillary Clinton, was the one who started talks to give 400 million dollars, in cash, to Iran. Scandal!" Trump said in a Twitter post.

Republican National Committee spokesman Reince Priebus also weighed in. "The Obama-Clinton foreign policy not only means cutting a dangerous nuclear deal with the world's number one state sponsor of terrorism, it also means paying them a secret ransom with cargo planes full of cash," he said in a statement.

House Speaker Paul Ryan was more measured, saying that: "If true, this report confirms our longstanding suspicion that the administration paid a ransom in exchange for Americans unjustly detained in Iran." – Reuters