Sen. Mark Kirk, R-Ill., referred to President Obama this week as a "drug dealer" as he blasted the White House for what Republicans are calling a $400 million ransom payment to Iran in return for four Americans.

The release of four prisoners in January coincided exactly with the U.S. State Department's payment of $400 million to Tehran.

"We can't have the president of the United States acting like the drug dealer in chief," Kirk said this week in an interview with the State Journal-Register's editorial board, "giving clean packs of money to a ... state sponsor of terror.

"Those 500-euro notes will pop up across the Middle East," he added. "We're going to see problems in multiple [countries] because of that money given to them."

The State Department said this week that the payment was used as "maximum leverage" to get Tehran to agree to the prisoner release.

"The payment of the $400 million was not done until after the prisoners were released," State Department spokesman John Kirby told reporters.

He avoided using the word "ransom," but he conceded the payment was "connected" to Iran agreeing to release its American prisoners.

"We, of course, sought to retain maximum leverage until after American citizens were released, and that was our top priority," he said. "If your top priority is to get your Americans out and you're already having some issues about locating some of them, [then] you want to make sure that that release gets done before you complete that transaction."

The State Department first announced the $400 million payment in January, explaining at the time that it had agreed to settle with Tehran over an outstanding claim at the Iran-U.S. Claims Tribunal in The Hague.

The more eyebrow-raising details of the payment, including that a plane was used to ship in hard currency, did not become widely known until the Wall Street Journal broke the story in early August.

"Wooden pallets stacked with euros, Swiss francs and other currencies were flown into Iran on an unmarked cargo plane, according to ... officials. The U.S. procured the money from the central banks of the Netherlands and Switzerland," the paper reported.

Though the Obama administration maintains there was no quid pro quo, the Wall Street Journal reported, "U.S. officials also acknowledge that Iranian negotiators on the prisoner exchange said they wanted the cash to show they had gained something tangible."

Iranian officials have also reportedly referred to the cash payment as a "ransom," according to the country's new services.