Washington secretly airlifted $400 million to Tehran amid a prisoner swap in January in which seven Iranians detained in the US were exchanged for Washington Post reporter Jason Rezaian and three other Iranian-American prisoners, The Wall Street Journal reported on Wednesday.

Administration officials denied that the $400 million cash transfer was tantamount to critics' assertion of a ransom payment, insisting that the funds were part of a $1.7 billion financial settlement the US had reached with Iran as part of the nuclear deal.

"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," State Department spokesman John Kirby told The Journal.

"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," he added.

But the fact that President Barack Obama failed to disclose the transfer of foreign hard currency to Iran via an unmarked cargo plane has once again raised questions about the administration's transparency in its dealings with Iran.

"We were right in January 2016 to describe the administration's $1.7 billion transfer to Iran as a ransom payment," US Sen. Mark Kirk, a Republican from Illinois, said in a statement on Wednesday.

He continued:

"Paying ransom to kidnappers puts Americans even more at risk. 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, including Siamak Namazi, his father Baquer Namazi, and Reza Shahini."

Amir Toumaj, an Iran expert at the Washington, DC-based think tank Foundation for Defense of Democracies, noted in an email that neither the prisoner swap nor the payments seem to have deterred Iran from detaining Americans.

"Instead of holding the regime and IRGC accountable for their actions, they have been rewarded," Toumaj told BI. "The Guard has arrested six dual nationals since January, and dozens of unlucky Iranians who do not have Western passports."

View photos United States Secretary of State John Kerry (L) meets with Mohammad Javad Zarif, Minister of Foreign Affairs of Iran, at the United Nations in New York, September 26, 2015. REUTERS/Stephanie Keith More

The prisoner swap itself fueled skepticism about the extent to which the US may have appeased Iran in the name of finalizing the landmark nuclear deal. The exchange was later revealed to have been a year in the making, despite claims by administration officials that the subject of a prisoner release was never broached during nuclear-related negotiations.

The swap was also finalized on the same day the nuclear deal was implemented and the nuclear-related sanctions on Iran were lifted — a "coincidence," as officials characterized it, that left some experts wondering whether the exchange was aimed at bolstering overall support for the deal inside Iran.

"The administration announced the transfer of the $1.7 billion, of which the $400 million is the first installment, in January," Toumaj said. "Still, the timing of this first installment is suspicious," he added. "And it’s difficult to imagine that the prisoner swap and money transfer were not part of a quid pro quo arrangement."

The negotiations leading up the exchange were apparently kept so far under wraps that even some Washington officials and insiders were not aware they were happening. The secrecy surrounding the issue fueled one of the biggest criticisms of the Iran deal — namely, that it did not force Iran to release its American prisoners even as Tehran was due to receive sanctions relief.

Story continues