"This is harassment and intimidation of a kind that is quite unacceptable," British Foreign Secretary David Miliband told reporters during a security conference on the Greek island of Corfu. "We want to see (them) released unharmed."
An Iranian news agency reported on Sunday that eight Iranian employees of Britain's embassy in Iran had been detained for involvement in the post-election unrest in the Islamic Republic.
Miliband said the total number of employees in question had been about nine, but some had been released.
"We are still concerned about a number of them who to our knowledge have not been released … The numbers are changing hour by hour," he said.
"These are hard-working diplomatic staff and the idea that the British Embassy is somehow behind the demonstrations and protests that have been taking place in Tehran in recent weeks is wholly without foundation."
Miliband said Britain had made a strong protest to the Iranian authorities and he would discuss the issue with European Union counterparts in Corfu who were due to discuss the post-election situation in Iran on Sunday.
Iran has accused Western powers — Britain and the United States in particular — of interfering in its internal affairs after the vote, which sparked days of huge demonstrations in which at least 20 people were killed.
British Prime Minister Gordon Brown announced on June 23 that Britain was expelling two Iranian diplomats after the Islamic Republic forced two British diplomats to leave.
(Reporting by David Brunnstrom; Editing by Richard Williams)
