New details about a $400 million cash payment to Iran in January just as Iran released four U.S. hostages is raising fears among advocates for current hostages that Tehran is upping the stakes, and will only release them after additional concessions.

Iran issued indictments last month against four people with ties to the U.S., including an American with dual Iranian citizenship, and a U.S. permanent resident.

All four were believed to be picked up over the last year by Iran's Revolutionary Guard, a hardline military unit devoted to maintaining the country's Islamic system, during what human rights activists say has been an uptick in incarcerations of dual-Iranian citizens or people with western ties.

At least one of the four, businessman and information technology expert Nizar Zakka, 49, is being held in long-term solitary confinement under vague charges that include "collaborating with hostile governments," according to an urgent action notice by Amnesty International.

Zakka is a Lebanese citizen and permanent resident of the United States who has lived in Washington, D.C., for years. The three others indicted in July were Siamak Namazi, an Iranian-American businessman; Homa Hoodfar, an Iranian-Canadian and retired professor at Montreal's Concordia University; and Nazanin Zaghari-Ratcliffe, an Iranian-British woman who works for Thomson Reuters Foundation, the news outlet's charitable side.

Zakka was arrested in Iran last September after traveling there to attend an International Conference and Exhibition on Women in Sustainable Development, at the invitation of an Iranian official to serve as one of the events speakers, the alert said.

His lawyer, Jason Poblete, says Zakka's health has been deteriorating in recent months and he is on a hunger strike to protest his harsh treatment.

"The family is concerned about his well-being and they are asking the U.S. government and anybody who can to please help secure his release," Poblete, an attorney and former senior Republican congressional staffer, told the Washington Examiner.

"He needs medical attention — a doctor has not been allowed to go in," he said. "They are doing things to him that are psychologically and mentally distressing, and the family just wants him released. They want medical attention but they want him released unconditionally on humanitarian grounds."

The case is particularly relevant to the State Department because Zakka was reportedly working on a U.S. government grant when he visited Iran. According to an Associated Press article published in May, Zakka has received $730,000 worth of grants from the U.S. government for projects in the region. Amnesty International said Zakka helped to set up a "regional alliance of information and technology organizations" across 13 companies in the Middle East and North Africa region.

But reports that the U.S. sent Iran a $400 million cash payment the same day American prisoners were sent home has Poblete worried that the payment was effectively a ransom, and that Iran might seek more concessions before Zakka is released, including possibly concessions related to the Iran nuclear agreement.

"We hope that our government is not salvaging the [nuclear deal] on the backs and lives of these innocent people, including Mr. Zakka," Poblete said.

Amnesty International's Elise Auerbach, an Iran country specialist, said she also notes more detentions by Iran.

"We are concerned about all people who are being held for political reasons in Iran but we also have concerns about the increase in harassment of dual nationals and people who have ties to countries outside of Iran," she said.

Zakka was in Iran at the invitation of the Iranian government itself, and Auerbach said the detainees appear to be the result of a split within Iran over how much to open up to the West.

"Members of Iranian President [Hassan] Rouhani's administration and Rouhani himself have tried to put out the welcome mat for ex-patriots and Iranians and have actively tried to promote business ties and academic and cultural and scientific ties," Auerbach said.

But hardliners in Iran who answer to the supreme leader are stymieing those efforts, she said. "[T]hey appear to be on a campaign to undermine [Rouhani's efforts] to open up to the West … and they are trying to create an atmosphere of fear."

The Obama administration is insisting that the lump sum payment in January was not ransom but part of a separate agreement to compensate Iran for money it paid to buy U.S. jets, a deal that never happened after the Shah of Iran was overthrown in 1979.

But Republicans are not convinced. Rep. Ed Royce, R-Calif., who chairs the House Foreign Affairs Committee, is incensed over details of the payment because he has been asking for them since February of this year when he first sent a letter to Secretary of State John Kerry. Kerry did not fully respond to the request, he said this week.

"The logistics of this payment – literally delivering a plane full of cash to evade U.S. law – shows yet again the extraordinary lengths the Obama administration will go to accommodate Iran, all while hiding the facts from Congress and the American people," he said.

Looking ahead, a House Foreign Affairs Committee staffer confirmed to the Examiner that Royce has specifically asked the State Department for more information about Zakka's and other prisoner's cases.

White House press secretary Josh Earnest on Wednesday sidestepped a question about whether the Iran deal had anything to do with the detainment of current hostages, saying instead that their safe return is a "top priority" of the U.S. government.

"The status of Americans held against their will around the world is a top priority of the U.S. government and there are significant efforts and significant resources that are committed to seeking the recovery of Americans who are unjustly held or held hostage around the world," he said, noting that privacy laws legally prevent him from speaking about individual cases.

That matches recent comments from the State Department, which has said the U.S. is always raising issues related to people detained by Iran. But in April, department spokesman John Kirby said Zakka's lack of U.S. citizenship prevents the government from pushing for his return through its consular service.

"On this particular case, we are concerned about the case of Mr. Zakka, a Lebanese citizen and U.S. lawful permanent resident, who's been unjustly held in Iran since September of 2015," Kirby said. "The Immigration and Nationality Act prevents us from providing consular assistance to non-U.S. citizens. Consular assistance would be provided by the country of the individual's nationality, and I don't have any additional comments to make on this particular case."

However, the State Department's own Foreign Affairs Manual, state that the U.S. government has wide discretion in advocating for legal permanent residents.

"At times, you will come across arrest cases of individuals who are not U.S. citizens or nationals but who are legal permanent residents with strong ties to the United States," the manual states. "Their arrest may come to your attention from other family members in the United States, other prisoners, congressional offices, or even host government officials who on occasion are not quite clear on the exact status of a U.S. 'green card' holder."

"The department's general guidance in such cases is: While consular officers do not have the right to demand consular access and visitation for U.S. Lawful Permanent Resident Aliens (LPRs), they may do so on a courtesy basis," it states.