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 Tehran 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."

Noting that no sentence had been announced, Baron 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.

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.