The family of former FBI agent Robert Levinson and the relatives of nearly a half dozen others held captive in Iran say they “shall remain quiet no longer” about demands for world governments to help secure the release of those hopelessly detained in the Islamic Republic.

The declaration came in an open letter addressed to “World Leaders, Rights Organizations and Media Outlets” that was published by the group earlier this week. The families have “banded together now to come to you as one voice," the letter stated.

“We believe that the Iranian authorities have little incentive to end the cruel and horrific practice of hostage taking as a result of inadequate pressure from the international community,” the letter reads. “World leaders need to make the political cost for committing human rights violations so high that releasing our loved ones becomes advantageous to the Iranian authorities.”

The letter highlighted the plights of six dual and foreign nationals who have been held hostage in Iran: Ahmadreza Djalali, Kamran Ghaderi, Nizar Zakka, Saeed Malekpour, Siamak and Baquer Namazi and Levinson. The latter four are some of the nearly 20 known American hostages who remain in captivity or who have been imprisoned by hostile regimes.

Zakka, a Lebanese-born Internet freedom activist who is a permanent resident of the U.S., was detained in Iran in September 2015 while attending a woman’ empowerment conference he was invited to. He has been sentenced to 10 years in prison on spying charges.

A month later, Siamak Namazi, an Iranian-American businessman, was nabbed while visiting his family, three months after the Obama administration-led Iran nuclear deal was signed.

Namazi's father, Baquer, a former UNICEF diplomat, was arrested in February 2016 after the Iranian authorities granted him permission to visit his son in the notorious Evin Prison in Tehran.

And Levinson, now believed to be the longest-held American hostage in history, was 58 when he boarded a flight from Dubai and then made his way to Kish Island, a resort in the Persian Gulf, in 2007. He was allegedly investigating cigarette smuggling and possibly working on a book, but after checking into the Maryam Hotel, he met with an American fugitive, Dawud Salahuddin, also known as David Belfield and Hassan Abdulrahman.

Salahuddin, who is still wanted for the 1980 murder of an Iranian diplomat in Maryland, was being targeted by the CIA for recruitment, a source close to the Levinson case told Fox News in 2016. Levinson was hoping a successful mission that delivered Salahuddin would lead to full-time hours with the CIA, the source said.

The Society of Former Special Agents of the FBI, which includes Levinson as a member, told Fox News that it feels like he is being used as “a bargaining chip for a hostile foreign power."

“I think we need to commend the Trump administration for really preaching hard on this and realizing that Iran is a hostile foreign power and that they continue to take these hostages,” added the society's executive director, Nancy Savage.

The FBI said earlier this year that the bureau and “our partners in the United States Government have worked tirelessly to bring Mr. Levinson home” – and a $5 million reward has been offered for Levinson's safe return.

But Levinson’s family -- and the others who wrote and signed the letter -- say more needs to be done.

"Over several agonizing years, our loved ones’ cases have each been treated individually, but while they are all unique and complicated cases, this is not an individual problem, it is a pattern; a pattern we call on world leaders to help end," the group said in its letter. "Responsible stakeholders on all sides of this issue know what to do. Please, secure our loved ones’ freedom from Iranian prisons."

The relatives added: “We are a determined group of individuals and families speaking to our governments with one voice, as human beings who have been affected by the horrific situation that the Iranian authorities have created. We are asking for action."

Richard Grenell, the U.S. Ambassador to Germany who previously was the longest-serving U.S. spokesman at the U.N., said this week Levinson’s family deserves information on his conditions and whereabouts. Levinson last surfaced in a 2011 video clip.

“I encourage the public to demand the release of all these hostages,” he said, referencing the open letter. “We will not rest until Mr. Levinson, and the rest of Iran’s hostages, are safely home with their families.”

Fox News’ Hollie McKay, Jennifer Griffin and Lucas Tomlinson contributed to this report.