In an appearance on CNN’s “Reliable Sources" with Brian Stelter on Sunday morning, Washington Post Executive Editor Martin Baron said the paper is doing everything to free its reporter in Tehran, who has been in prison for nine months.

Baron has been rallying for the release of The Post’s Tehran bureau chief Jason Rezaian, who has been charged with espionage and other crimes, including collecting classified information.

“There has been no evidence provided by the Iranian government that he engaged in espionage or did anything other than report on what was happening in that country,” Baron said. “In fact, most of his coverage focused on the culture and daily life of people in Iran.”

[Post reporter jailed in Iran faces 4 charges including espionage]

Baron said The Post is talking to U.S. officials, who have had conversations with the Iranian government about getting Rezaian released. The paper is also pleading its case to other governments in the region and elsewhere, but Baron said there are no prospects for an imminent release.

Rezaian’s imprisonment came up at the White House Correspondents’ Association dinner Saturday night, with President Obama saying the government would not rest until the journalist is freed.

President Obama and other attendees of the 2015 White House correspondents' dinner call for release of Jason Rezaian, the Washington Post correspondent in Tehran, who has been in Iran's Evin Prison since July 2014. (AP)

“For nine months, Jason has been in prison in Tehran for nothing more than writing about the hopes and fears of the Iranian people,” Obama said.

The president told the room full of reporters, many of whom were donning “Free Jason” pins, that he had spoken with Rezaian’s brother, Ali, who was at the dinner.

“I have told him personally that I will not rest until we bring him home to his family, safe and sound,” Obama said.

The president is among a growing chorus of government officials, including Secretary of State John F. Kerry, urging Iran to free Rezaian.

A group of Republican senators pressed the administration to make Rezaian’s release a condition for a nuclear agreement with Iran. But the administration has been averse to tying the imprisonment with the effort to block Iran’s ability to build nuclear weapons.

Rezaian, 39, has been a reporter for The Post since 2012. He and his journalist wife, Yeganeh Salehi, were arrested July 22. She was later released on bail, but Rezanian has remained in custody.

The Revolutionary Court, the venue for national security cases, took up the couple's cases. Although the court has never publicly disclosed the charges against Rezaian, his attorney has said that the charges stem from inquiries and contacts he made as a journalist.

Asked about the motivation for the arrest, Baron said, “There has been plenty of speculation that there is some sort of conflict between the Revolutionary Guard and the government of President Rouhani and his foreign ministers, but we don’t know that for sure. We’re not in a position to speculate. We’re mystified by it.”

No date has been set for the trial, but Rezaian’s attorney has said that it may begin in the next few weeks. A judge known for imposing harsh sentences, including the death penalty for anti-government protesters, is slated to hear the case.

[Hard-line judge in Iran is assigned case of jailed Post reporter Jason Rezaian]

“It’s a terrifying situation and an entirely unjust situation,” Baron said of Rezaian’s imprisonment. “There is absolutely no justification for this happening.”

Baron said Rezaian has been held longer than any other journalist in Iran. The reporter has spent much of the nine months in isolation, where he has suffered health problems and bouts of depression. Baron said Rezaian has been denied the treatment he needs and was granted time to meet with his attorney only in the past week, for 90 minutes.

“That is not a system of justice. That is a system of officially sanctioned injustice. And not a shred of evidence has been provided that he has done anything wrong,” Baron said.

Jason Rezaian's journey has taken him from a childhood in San Francisco to his father's native Iran. At 37, he became the Washington Post correspondent in Tehran. In July 2014, he was thrown into Iran's Evin Prison, where he remains. This is his story. (Jorge Ribas/The Washington Post)