The Washington Post denounced the conviction of its reporter Jason Rezaian by an Iranian court, saying the proceeding was an "outrageous injustice.''

In a statement issued on Tuesday, Washington Post Executive Editor Martin Baron asserted that Rezaian is innocent and Iran's conviction of him in a closed-door trial on Sunday was "unclear" and his verdict "unconscionable,'' according to Fox News.

"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,'' Baron said.

The journalist's fate was not immediately known as the Iranian judiciary spokesman Gholam Hossein Mohseni Ejehi did not clearly state whether the court's decision was a conviction. The press conference held Sunday was reportedly restricted to Iranian media, said Fox News.

"He has been convicted, but I don't have the verdict's details," Ejehi told state TV Sunday. He said Rezaian and his lawyer are eligible to appeal the conviction within 20 days.

Leila Ahsan, Rezaian's lawyer, told reporters that she was unaware that the verdict had been announced. "I have yet to receive the decision.''

The verdict on Rezaian's case comes 15 months after he and his wife were detained by Iranian authorities for alleged spying. His wife and two other journalists had already been released.

A dual citizen who was born and raised in California, Rezaian has spent most of the time since July 2014 in solitary confinement in the notoriously brutal Evin Prison without access to a lawyer. He is also suffering from some unspecified health problems.

Iran earlier slapped multiple charges, including espionage, against the Washington Post Tehran Bureau chief.

His trial has been widely criticised by the US government and press freedom organisations.

The Washington Post said it is working with the Rezaians and its legal counsel to swiftly appeal the verdict and press for his release on bail pending a final decision.

The US State Department, meanwhile, said the conviction of Rezaian was "not surprising.''

"We've seen news reports that Jason has been convicted, but we still have not seen any official confirmation of a verdict on specific charges or any further information,'' said State Department spokesman John Kirby.

"Unfortunately, this is not surprising given that this process has been opaque and incomprehensible from the start. Regardless of whether there has been a conviction or not, we continue to call for the government of Iran to drop all charges against Jason and release him immediately,'' he added.