The BBC has complained to the United Nations after Iran has began a criminal investigation into more than 150 of the corporation's Persian staff accusing them of a "conspiracy against national security" in Iran and abroad.

Iran has imposed an asset freeze on BBC Persian staff, meaning they and their families cannot inherit family assets, or buy or sell properties or cars in the Middle Eastern nation, said the media outlet.

Director general of the BBC Tony Hall said in a statement: "This is the latest in a sustained campaign of harassment and persecution which is designed to pressure journalists against continuing their work for the BBC."

As part of this campaign the families of BBC Persian staff in Iran have been subject to arrest and detention, confiscation of passports, ongoing surveillance and harassment, the said the media organisation.

Hall added: "The Iranian government is conducting what appears to be a politically motivated investigation into 152 BBC Persian staff, former staff and contributors, accusing them of conspiracy against national security in Iran.

"This is an unprecedented collective punishment of journalists who are simply doing their jobs."

Iran has accused the British media outlet of stirring unrest after the disputed re-election of President Mahmoud Ahmadinejad in 2009, adding that its reporters were working against the interests of the Islamic Republic.

The BBC has complained to David Kaye, the UN special Rapporteur on the promotion and protection of the right to freedom of expression and Asma Jahangir, UN Special Rapporteur on human rights in Iran.

Britain's National Union of Journalists (NUJ) backed the BBC's complaint.

NUJ general secretary Michelle Stanistreet said: "The Iranian government has filed criminal charges citing conspiracy against Iran's national security. These ludicrous charges amount to the collective punishment of journalists and a crass attempt to intimidate."

BBC Persian has broadcasts TV and radio news in Persian to Iran and the Persian speaking world from London since 2009, reaching an estimated audience of 18 million a week.