Five Americans are believed to be held as prisoners in Iran, and the family of one says he is pleading to President Trump -- and all of us – to help.

"I ask myself and my fellow American neighbors: Where is the justice I have come to associate with America?" Robin Shahini, 46, wrote to his family from an Iranian jail.

Shahini was convicted of collaborating with "a hostile government,” the U.S – an accusation his family denies. He was reportedly sentenced to 18 years in prison.

"This charge is unjust and the Iranian government intended to commit this wrong against me, an innocent American citizen, for political purposes. I ask of you, please to not let Iranian government use me," Shahini wrote in his letter.

"I ask you beloved citizens and all human-loving individuals to not leave me alone and defend my rights, which is also the right of each and every one of you. Defending me is defending yourselves. Do not let me be alone."

SAN DIEGO MAN REPORTEDLY DETAINED IN IRAN

Shahini is a graduate student from San Diego who last year traveled to Iran, he says, to visit his ailing mother who suffers from Alzheimer’s disease. In June, just eight days before he was scheduled to fly home, his family said he was arrested at his mother's home, accused of having once met with the Iranian opposition and criticizing the government on Facebook.

His family said he went on a hunger strike on February 15th. They are calling for his release, fearing that he could die behind bars.

"I ask him to end his hunger strike," Shahini's sister Fatemeh, told Fox News. She calls his sentence "an unjust conviction," and said "after changing the conditions between Iran and the United States, they did worse to my brother."

Shahini's girlfriend, Sevil, who does not want her last name used, said he told her that "they treat him like a terrorist, calling him ‘foreigner,’ ‘American,’ in a very insulting way."

"He is a hostage because he is American. They see him as a hostage and they really treat him that way."

"They are silently killing him," she says.

The number of arrests and detentions of visitors…especially dual-citizens… has spiked, warns Lisa Daftari, the editor of the website "The Foreign Desk," who has followed Shahini's case.

"In the aftermath of the nuclear deal with Iran, we would expect things to get better," she said. "But we've seen an increase in executions, we've seen an increase in crackdowns against journalists, against dual-citizens, against academics, political dissidents, women's human rights leaders. And this is not what we expected."

Daftari also said the arrest and trial of Shahini, and other dual-U.S. citizens like him, serves as a broader propaganda purpose for Tehran.

"The Iranian regime is delivering a stern message to Iranians living abroad, not to get involved in political activity, not to speak out against the regime, and they want Iranians to know that they are in fact being watched."

IRAN CONFIRMS IRANIAN-AMERICAN HAS BEEN DETAINED

Meanwhile, Shahini's loved ones wait and hope.

"I am asking all the U.S. citizens and again the U.S. administration to help, as I think he is not going to last long with this situation with all health issues going on," Sevil said. "I'm really demanding that the U.S. administration take action."

White House Press Secretary Sean Spicer told Fox News that "obviously we are aware of the situation and would refer you to the State Department," when asked about Shahini’s plight at the White House briefing.

The State Department told Fox News that it is also aware of the reports, and continues to use all the means available to advocate for U.S. citizens who are unjustly detained overseas.

The U.S. has issued a travel warning for citizens, and dual citizens, to Iran, warning of the detentions and arrests. Iran does not recognize dual citizenships, which makes the prospect of prosecution greater.

Daftari said a strong message could be sent about the imprisoned Americans from the White House.

“I think President Trump has a great opportunity right now to reach out to the people of Iran, whether they are living in the United States or Iran, to express to them that he understands their human rights abuses, and to do whatever he can in his position to free these prisoners and to improve the human rights conditions in Iran.”

Fox News has requested comment from Iran's Mission to the United Nations but there has yet to be any comment.

Ben Evansky contributed to this report.