U.S. White House national security adviser Jake Sullivan speaks at a press briefing at the White House in Washington, U.S., December 12, 2022. REUTERS/Kevin Lamarque/File Photo

Iran has arrested and detained a fourth American national, creating an additional complication in negotiations for a US-Iran prisoner swap.

As first reported by Semafor, three people briefed on the case say that the fourth arrest is now central to negotiations to release Americans held in Iranian custody in exchange for Iranians held in Western prisons. The report does not name the fourth American detainee. Iran is also known to be holding two US permanent residents.

US National Security Advisor Jake Sullivan in an interview on Sunday with CBS’s Face the Nation implicitly confirmed that Iran was now holding four US nationals.

“We have tried very hard to secure the release of the four unjustly detained Americans in Iran, we have done so since the day that President Biden took office,” Sullivan said. “We have had indirect contact with Iran on this in an effort to try to get a deal that could get them released. We have not arrived at an understanding that would get them out at this point. We are continuing to work at it. My hope is that we will get it done.”

The US and Iran have been holding so-called “proximity talks” on the detainee issue and Iran’s nuclear program hosted by Oman in which the negotiators exchange messages through Omani intermediaries. Sullivan added in the interview that the US is “not close to any kind of a deal” on Iran’s nuclear program.

Iran has a decades-long history of using foreign detainees as a bargaining chip. While officially kept as separate negotiations, the implementation of the 2015 nuclear deal coincided with a same-day release of American prisoners and the delivery to Iran of hundreds of millions of dollars worth of cash. Critics of the deal including Sen. Tom Cotton (R-AK) said at the time that the payments amounted to a ransom and would encourage Iran to take additional hostages.

While Iranian officials have suggested that a prisoner exchange could be imminent, it has also taken increasingly aggressive actions in recent months. Earlier in July, Iran attempted to seize two oil tankers in the Gulf of Oman, which was prevented by the US Navy. The Department of Defense on Thursday announced that it was deploying additional ships and a Marine expeditionary unit to the region to deter Iran.

“Through these actions, the United States is demonstrating commitment to ensuring freedom of navigation and deterring Iranian destabilization activities in the region,” the Department of Defense said in a statement.