Speaking during a visit to Madrid, Iran's top diplomat denied that journalist Jason Rezaian, who reportedly faces espionage charges, was being mistreated as alleged by his employers.
Iranian authorities have provided him with "humanitarian" assistance, including the right to receive visits from his mother and other relatives, Zarif said in response to a question about the case at a press conference.
"On the legal issue, this is a judicial matter," he went on.
The Iranian judiciary "believe that the charges against him are very serious", Zarif said, without giving details.
While the charges have yet to be revealed the minister was adamant that the case would go to trial, would be free of political interference, and that the accused, who is the Post's correspondent in Iran, would have full access to his lawyer.
His remarks came after The Washington Post on Tuesday accused Tehran of thwarting Rezaian's access to legal counsel.
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At his "only meeting" with his lawyer Leila Ahsan "several weeks ago" they "were not permitted to discuss Jason's case or the charges he faces," the Post's executive editor Martin Baron said, rejecting claims by Ahsan, who has said she can meet with her client "anytime".
Rezaian, a dual US and Iranian citizen, was detained in Tehran in July 2014. He is being held in the city's notorious Evin Prison.
The charges against him have not been made public but Iran's Fars news agency said Sunday he would be tried on charges of espionage and crimes against national security.
Fars, which is believed to be linked to Iran's elite Revolutionary Guards, said the journalist was accused of "selling Iran's economic and industrial information" to the US.
The report came just days after Washington and Tehran reached a landmark framework agreement on curbing Iran's nuclear programme.
The Post, in its statement, said its man in Tehran "has done nothing wrong" and accused Iran of subjecting him to "harsh interrogation, months of solitary confinement, and to poor living conditions that have had a serious impact on his health."