As the Obama administration prepares for its next negotiating session with Iranian representatives in picturesque Vienna over Tehran's illicit nuclear weapons program, three Americans are languishing in Iranian custody.

Sadly, the U.S. has made no assertive effort to secure the release of the Americans – former FBI agent Robert Levinson, Pastor Saeed Abedini and former U.S. Marine Amir Hekmati – or to tie their release to American concessions.

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Levinson recently became the longest-held hostage in U.S. history. Since his abduction on Iran's Kish Island in 2007, Iranian authorities have refused to disclose his whereabouts.

Abedini is serving an eight-year sentence for the crime of practicing Christianity. He has reportedly suffered internal injuries in prison, and Iranian authorities refuse to provide him crucial surgical care for his condition.

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Hekmati, who was convicted of espionage while visiting his grandmother in Iran and sentenced to death, managed to smuggle a letter from prison to Secretary of State John Kerry last year. He wrote that he is being held on false charges based solely on confessions obtained by force, threats, miserable prison conditions and prolonged solitary confinement. An Iranian court has reversed his death sentence, but no new trial date has been set.

Unsurprisingly to human rights observers, Iran was recently fingered by Dr. Ahmed Shaheed, the United Nations special rapporteur on the situation of human rights in Iran, as a country with a “lack of adherence to the rule of law.” According to his most recent report, “religious minorities, including recognized Jews, Christians and Zoroastrians, also face discrimination in the judicial system, such as harsher punishments.” This, of course, is happening under the tenure of the so-called moderate presidency of Hassan Rouhani.

To be fair to President Obama, he raised the imprisonment of Pastor Abedini in his phone conversation with Rouhani last September. But the talks appear to have reached a dead end. Iranian intransigence remains as robust as ever, and the specter of a prisoner release appears to be no further along today than it was before the president made his historic phone call to Rouhani.

Tehran does not miss opportunities to needle Washington as it pursues greater economic concessions from the West and the release of Iranians being held abroad on security-related charges. 

Late last month, news reports surfaced that Iran would appoint Hamid Aboutalebi, a member of a radical Islamic group that seized 52 U.S. embassy employees as hostages in 1979, as the country’s new ambassador to the U.N. It is unclear at this time whether the Obama administration plans to approve Aboutalebi’s visa application.

The appointment of Aboutalebi prompted justifiable outrage from former U.S. hostage Barry Rosen, who said it would be an “outrage” and “disgrace” if Washington green-lighted his visa application.

Because easing economic sanctions is at the forefront of policy priorities in Tehran, the U.S. should take advantage of its opportunity to sit across the table from Iranian negotiators to demand the release of the three Americans. Should Iran balk, the U.S. should pursue robust human rights sanctions targeting all officials involved in their incarceration.

Most importantly, the U.S. should make the release of the three Americans a sine qua non of the start to next week’s nuclear talks. As Levinson’s son Dan has urged the Obama administration, bringing his father home "should be the first topic of any discussions with the Islamic Republic and prerequisite for any final deal related to its nuclear program.”

Doing so is probably the best hope the captives have of ever returning to American soil, and tying human rights abuses to the negotiations already underway might illuminate the thuggish character of the Iranian regime for anyone still clinging to the belief that moderates have been empowered.