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The Â鶹ԼÅÄ's director general has accused the Iranian authorities of intimidating those working for its Persian service.

that the Â鶹ԼÅÄ had seen "disturbing new tactics", including the targeting of family members of those working outside Iran.

Last week, the sister of a Â鶹ԼÅÄ Persian staff member was detained and held in solitary confinement at a Tehran jail.

Iran accused the Â鶹ԼÅÄ of inciting unrest after the disputed re-election of President Mahmoud Ahmadinejad in 2009.

Â鶹ԼÅÄ Persian broadcast online videos and interviewed protesters, who described deaths, injuries and arbitrary arrests carried out by security forces.

In his blog, Mr Thompson wrote that for Â鶹ԼÅÄ Persian staff, "interference and harassment from the Iranian authorities has become a challenging fact of life".

"In recent months, we have witnessed increased levels of intimidation alongside disturbing new tactics," he added.

"This includes an attempt to put pressure on those who work for Â鶹ԼÅÄ Persian outside Iran, by targeting family members who still live inside the country."

Mr Thompson revealed that last week the sister of a Â鶹ԼÅÄ Persian member of staff was arrested and held in solitary confinement on unspecified charges at Evin Prison in the capital, Tehran.

"Although she has now been released on bail, her treatment was utterly deplorable and we condemn it in the strongest possible terms."

Human Rights Watch said that at one point, a man claiming to be the relative's interrogator at Evin telephoned the staff member in London and offered to release her in return for information about the Â鶹ԼÅÄ.

Mr Thompson said staff had also faced false accusations of sexual assault, drug trafficking and financial crimes.

In recent months, a number of relatives of Â鶹ԼÅÄ Persian staff had been detained for short periods of time by the Iranian authorities and urged to get their relatives in London to either stop working for the Â鶹ԼÅÄ, or to "co-operate" with Iranian intelligence officials, Mr Thompson said.

In other instances, passports of family members have been confiscated, preventing them from leaving Iran, he added.

"This has left many Â鶹ԼÅÄ Persian staff too afraid to return to the country, even to visit sick or elderly relatives."

He said some had also had their Facebook and email accounts hacked, and been subjected to a "consistent stream of false and slanderous accusations... in the official Iranian media, ranging from allegations of serious sexual assault, drug trafficking, and criminal financial behaviour".

Some reports claimed they had converted from Islam to Christianity or Bahai faith - potentially a capital offence in Iran as it is considered to be apostasy.

Human Rights Watch's Middle East director, Sarah Leah Whitson, said the actions of the Iranian authorities suggested they were attempting to silence reporters and the Â鶹ԼÅÄ, and were sending "a message that the government's long arm of repression can extend well beyond borders".

UK Middle East Minister Alistair Burt said Iranian officials' "deplorable tactics illustrate again the deteriorating human rights situation in Iran, and the desperation of the Iranian regime to silence any independent voices".