WASHINGTON - US President Barack Obama reiterated Thursday that $400 million that the United States helped airlift to Iran earlier this year was not a ransom for the release of American prisoners.

"Some of you may recall we announced these payments in January. Many months ago. They were not a secret," he told a press conference at the Pentagon, emphasising that the United States does not pay ransoms.

In January, five American prisoners were released as Washington granted clemency to seven Iranians and withdrew arrest warrants for 14 others.

Soon thereafter, the United States helped airlift $400 million worth of Swiss francs and euros to Iran.

The White House has said it was returning cash from a 1970s Iranian military order that was not fulfilled because of the Islamic Revolution.

"We do not pay ransom for hostages. We have a number of Americans being held all around the world. I meet with their families. And it is heartbreaking," Obama said.

On Tuesday the Wall Street Journal reported that the chunk of cash had been loaded on wooden pallets and secretly airlifted to Iran in an unmarked cargo plane.

Obama said that the timing of the delivery was "dictated by the fact that as a consequence of us negotiating around the nuclear deal, we had diplomatic negotiations with Iran for the first time in several decades."

Following the deal, Secretary of State John Kerry was able to "meet with the foreign minister, which meant our ability to clear accounts on a number of different issues at the same time converged".

Obama said nonpayment of the owed money posed a "significant litigation risk" that could have cost the United States billions of dollars.

Cash was used for the payment because the US does not have a banking relationship with Iran -- "precisely because we’re so strict in maintaining sanctions," Obama said.

"We could not send them a check. And we could not wire the money," he said. – AFP