LONDON, (Reuters) – Iranian authorities are increasingly arresting and threatening the families of British Broadcasting Corporation journalists to force them to quit its Persian news service, the BBC’s head said on Sunday.

Mark Thompson, director-general of the BBC, said he was speaking out to try and embarrass Tehran to end what he said was a campaign of intimidation against Iranian staff who worked outside Iran.

“This is a growing pattern. It’s systematic and a campaign,” Thompson told BBC TV.

“What we have decided to do is to be more public than we have been before in calling for the Iranian authorities to desist from this, to ask for other governments to try and put as much pressure as they can and to hope that the embarrassment of this will get those who are responsible for these actions to think again.”

He said on his blog: “In recent months, we have witnessed increased levels of intimidation alongside disturbing new tactics. This includes an attempt to put pressure on those who work for BBC Persian outside Iran, by targeting family members who still live inside the country.”

There was no immediate comment from Iranian authorities.

BBC Persian staff provide Farsi-language TV, radio and online services. Few Western journalists are permitted to work in Iran where the hardline Islamic government views much of the foreign media with suspicion.

The BBC’s TV service has often been jammed and is only available to owners of illegal satellite receivers.

Last September, Iran arrested several people for supplying information to the BBC, accusing them of seeking to portray a negative image of the Islamic Republic.

Thompson said the harassment was getting worse, citing the case of a sister of a London-based BBC journalist who he said was arrested in Tehran on unspecified charges, threatened and intimidated.

“She was quite clear this was absolutely associated with the fact that her sister was working for the Persian service in London,” he said.

Thompson said such pressure was designed to force people to resign from the BBC or become informants for the Iranian intelligence service.

Despite attempts by Tehran to prevent access to the BBC, he said Iranians were very ingenious at finding ways of reaching the service. “We know from the extraordinary feedback we get (that) it’s watched and relied upon by many, many millions of people inside Iran,” he said.