Three journalists -- including the Washington Post's Tehran correspondent -- arrested in Iran were detained for security reasons, the country's prosecutor said in a statement published on Monday.
It was the first time an Iranian official has given a reason for the July 22 arrest of the Post's Jason Rezaian and a photographer, both of whom are dual US-Iranian citizens.
Rezaian's wife Yeganeh Salehi, also a journalist, was detained the same day.
"Three people were arrested in this case. It is a security case," Gholamhossein Mohseni-Ejeie, who is also the chief spokesman for the Iranian judiciary, said in a statement published by ISNA news agency.
"In line with the law I cannot reveal the details of the case or the charges facing the accused," he said," adding that an investigation was under way.
Previously, chief justice Gholamhossein Esmaili had hinted at espionage but made no specific allegations.
Iran's Deputy Foreign Minister Hassan Ghashghavi said earlier this month the detention of Rezaian and the photographer -- whose family have asked that her identity not be disclosed, was a domestic issue.
"We do not accept dual nationalities. If a person enters Iran with an Iranian passport, that person is considered an Iranian citizen," Ghashghavi told Vatan-e-Emrooz, a prominent conservative daily.
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"And we do not allow other countries to demand consular rights on behalf of Iranians," he said.
Rezaian's wife Salehi is Iranian and works for the United Arab Emirates newspaper The National. She is said to have recently applied for permanent US residency.
The United States, which has no diplomatic ties to Iran, has called for the journalists to be freed. The Washington Post, the United Nations and media watchdogs made similar appeals.
A State Department spokeswoman said America had reached out to the Swiss embassy in Tehran, which acts as a go-between, to try to ensure consular officials can visit Rezaian.
Foreign observers have told AFP the arrests could be linked to power struggles between Iranian conservatives who dominate the judiciary, and President Hassan Rouhani who is considered a moderate.
Rouhani advocates a more liberal view on implementing social freedoms in the Islamic republic, and began his presidency by restarting nuclear talks with the West.
His policies have triggered criticism from hardline clerics and conservatives.
Earlier this month, United Nations human rights experts expressed alarm at a recent flurry of arrests and harsh sentences inflicted on journalists in Iran.
They said that since May 22 more than 30 people, including journalists, bloggers, filmmakers and authors, have been arrested, summoned or sentenced in connection with journalistic activity or for expressing their opinions on social media.
Observers said they fear that the arrests of the journalists could undermine the thorny negotiations between Iran and the West for a comprehensive nuclear deal that world powers are seeking to reach by November 24.