US lawmakers Tuesday called on Tehran to immediately free four Americans held or missing in Iran after poignant testimony from relatives who described "living a nightmare" with little news and fading hope.

Some on the House foreign affairs committee and some family members urged the US administration to halt nuclear talks with the Islamic republic until the four are freed, or to make any deal contingent on their release.

As the committee voted on a bill to urge Iran to free the men, Representative Randy Weber paid tribute to the families, saying they were "real people, with real lives in real pain."

There should be "no agreement period, until Iran releases the hostages," he added, as others worried that once a nuclear deal is signed Washington will have less leverage to win the men's freedom.

Nagameh Abedini told how since her husband Saeed's 2012 arrest, "I have been carrying with me a deep excruciating pain knowing that my husband continues to suffer yet another day in one of the worst prisons in the world."

Saeed Abedini, a pastor, was sentenced to eight years in jail for gathering a group of people to study the Bible, charged with threatening national security.

He has been tortured, suffering internal bleeding and injuries, and beaten on his bare feet with a cable, Abedini said.

A few days after Iran and global powers reached a landmark accord on April 2 to guide the nuclear negotiations, her husband was returned to solitary confinement.

Ali Rezaian, the brother of Washington Post journalist, Jason Rezaian, whose trial opened last week shrouded in secrecy, denounced spying charges brought against his sibling as "false."

"It's time for the families here to all be reunited," he said, sitting next to a picture of his brother and a baseball cap from his beloved Oakland Athletics team.

- Dying wish -

The father of Amir Hekmati, a former US marine sentenced to death for alleged espionage after travelling to Iran to visit his grandmother, has terminal brain cancer. His dying wish is to hold his son again.

Hekmati's sister, Sarah, told the committee the family was struggling to understand "how previous American prisoners in Iran have been released when the United States had no diplomatic relations with Iran and were not sitting across from them at a negotiating table" yet Amir has remained in jail since 2011.

It "does not make much sense," she said, her voice catching with emotion.

Her brother, whose sentence has been reduced to 10 years in prison, is allowed to phone home every day for five minutes. But without any good news to tell him, the daily calls are almost a form of "psychological torture."

Robert Levinson, an ex-FBI agent who disappeared while on Iran's Kish island in 2007, is now the longest held hostage in American history.

Apart from a minute-long video sent to Levinson family in 2010 in which he looked gaunt and ill and a few subsequent pictures, they have had no news of him and do not even know where he is being held.

"Three thousand and seven days later we are still waiting for him to be released," his oldest son Daniel Levinson told the panel.

"Our family's hearts break for the other families here, who have suffered the wrenching agony of having their loved ones away from them for so long... Yet my father has been held four and a half years longer than any of the others."