WASHINGTON (AFP) - The Washington Post on Monday slammed the conviction of its reporter Jason Rezaian in Iran as an "outrageous injustice" and said it was working with his family and lawyer to prepare a quick appeal.

Rezaian, the US newspaper's Teheran correspondent and a dual Iranian-American citizen, was arrested in July 2014 and accused of spying, along with other crimes against national security.

The 39-year-old Rezaian was tried in four hearings behind closed doors, the last of which was held in August. A spokesman for Iran's judiciary, Gholamhossein Mohseni Ejeie, said the verdict that had been issued could be appealed.

"The guilty verdict announced by Iran in the trial of The Washington Post's Jason Rezaian represents an outrageous injustice," the paper's executive editor Martin Baron said in a statement.

"Iran has behaved unconscionably throughout this case, but never more so than with this indefensible decision by a Revolutionary Court to convict an innocent journalist of serious crimes after a proceeding that unfolded in secret, with no evidence whatsoever of any wrongdoing."

It was not immediately clear what Rezaian had been found guilty of. The charges against him are thought to carry a maximum penalty of 20 years in prison.

Baron, noting no sentence had been handed down, said the paper was "working with Jason's family and Iranian counsel to pursue an immediate appeal," adding that Rezaian's lawyer Leila Ahsan would petition for his release on bail while the case is still pending.

Rezaian's lawyer, wife and mother were "provided no further information" about the verdict "under the guise of a translator not being available" when they went to court Monday to seek clarification, his brother Ali said.

"Today's events are just the latest in what has long been a travesty of justice and an ongoing nightmare for Jason and our family," Ali Rezaian said in a statement. "This follows an unconscionable pattern by Iranian authorities of silence, obfuscation, delay and a total lack of adherence to international and Iranian law."

The treatment and trial of Rezaian has drawn condemnation from his family, the US government and press freedom groups as well as The Washington Post.

"The contemptible end to this 'judicial process' leaves Iran's senior leaders with an obligation to right this grievous wrong," Baron said.

"The only thing that has ever been clear about this case is Jason's innocence. Any fair and just review would quickly overturn this unfounded verdict. Jason should be exonerated and released," he said.

On Sunday, the US State Department said it was closely monitoring Rezaian's fate and again called for him to be freed.

Rezaian was arrested along with his wife Yeganeh Salehi, and a photographer. Salehi and the third individual were later released on bail, though Ahsan has said she expects Salehi to eventually face court action.