More than a dozen arrested Iranian journalists have been caught cooperating with a "Western-linked network" ahead of a presidential election in June, the intelligence ministry said on Wednesday.
Reports emerged this week that 14 journalists close to the marginalised reformists have been detained for alleged links to foreign Persian media, sparking an international outcry.
The intelligence ministry said in a statement carried by media on Wednesday that more arrests could follow.
The journalists belonged to "one of the biggest media networks" linked to the West, the statement said, adding that the charges against them are "completely solid and documented." It did not elaborate.
Their network, it said, was established by the BBC and operates "in cooperation with several Western governments."
The ministry said their goal was to "exploit what they learned during the sedition period" after the 2009 presidential election, which Iran accuses the West and Western media, including the BBC, of inciting.
The election, which returned President Mahmoud Ahmadinejad to power despite reformist opposition candidates alleging fraud, triggered protests which were met by a bloody regime crackdown.
On Wednesday, the intelligence ministry said the journalists had been under surveillance for months, and were found using "unique tools and methods to transmit news and receive directions" from abroad.
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It is possible more journalists will be "arrested or summoned in the coming days as investigations continue, or that some detainees are released, since they were probably unaware of links to abroad," the statement added.
Human Rights Watch and Amnesty International criticised the arrests, urging Iran to free the journalists.
"The arrests appear to be part of an escalating campaign of repression to silence journalists and bloggers before the presidential election scheduled for June 14," HRW said.
Amnesty called the arrests "a result of draconian restrictions on reporting which violate the right to freedom of expression and must be relaxed."
But the intelligence ministry dismissed such calls.
"It is certain that the will of ministry agents in carrying out their current task will be undeterred by the fuss created by the (Western) media, and that of their so-called human rights organisations," it said.
Those detained work for various reformist outlets such as Shargh, Arman, Bahar and Etemad newspapers, the Aseman weekly, as well as the ILNA news agency.
Iran's press watchdog has banned several publications, mostly reformist journals, for breaching its strict regulations since Ahmadinejad's re-election.
According to the Committee to Protect Journalists, 45 journalists were in Iranian prisons at the start of December 2012.