"The recent wave of arrests of journalists solely for carrying out their professional activities is a flagrant violation of Iran's obligations under international human rights law," four UN experts said in a joint press release.
Iran's intelligence ministry said last week that more than a dozen arrested Iranian journalists had been caught cooperating with a "Western-linked network" ahead of a presidential election in June.
Their network, it said, was established by the BBC and operators "in cooperation with several Western governments."
The UN experts said at least 17 journalists had been detained, and that arrest warrants and summons had been issued for several others.
The news was followed by an announcement from the intelligence ministry on Tuesday that a new round of arrests had been carried out.
An unknown number of journalists "have been arrested or summoned," since the previous wave of detentions, the ministry said in a statement cited by local media, referring to a warning earlier last week that more arrests could follow.
Iran's judiciary has yet to give any reasons for the arrests.
"After fully identifying the elements inside the country . . . all the foreign-based elements were also exposed," the statement said.
It claimed the network's headquarters, based in London, was "directly coordinating three units based in Brussels, Paris and Istanbul."
According to the statement, the majority of the external elements are journalists who fled the country after the 2009 disputed presidential elections, which also led to many arrests.
The UN experts said on Tuesday they were concerned that the latest wave of detentions was part of a broader campaign to crack down on independent journalists and media outlets in Iran.
"The right to communicate with international organizations, including non-governmental ones, is a fundamental aspect of freedom of expression, and using such accusations to conduct mass arrests flies in the face of Iran's international human rights obligations," the experts said.
"It is disturbing that mass arrests and detention are being used in retaliation against the exercise of freedom of expression," they said.
"Journalists must be able to speak and write without fear of persecution, arrest and intimidation."
The UN experts, Frank La Rue, Malick El Hadji Sow, Margaret Sekaggya and Ahmed Shaheed, respectively monitor freedom of expression, arbitrary detention, the treatment of human rights campaigners, and the overall situation in Iran.
According to the Committee to Protect Journalists, 45 journalists were in Iranian prisons at the start of December 2012.