Eight Iranian employees of the British embassy in Tehran have reportedly been arrested, accused of stoking post-election unrest in the Islamic Republic. The semi-official Fars News Agency has not named its source for the story, which is now being looked into by the Foreign Office. Foreign Secretary David Miliband has condemned Iran's "intimidation" tactics. "This is harassment and intimidation of a kind which is completely unacceptable," he said, adding that "strong action" would follow if the detentions did not stop. He went on: "The idea that the British Embassy is somehow behind the demonstrations and protests that have been taking place in Tehran... is wholly without foundation." Iran has accused Western powers, especially the UK and the US, of inciting street protests in the wake of a disputed presidential election on June 12. Fars said in its report: "Eight local employees at the British embassy who had a considerable role in recent unrest were taken into custody. This group played an active role in provoking recent unrest." A Foreign Office spokesman in London said: "We have in the last few days received a number of sometimes confused reports that British nationals or others with British connections have been detained. "We continue to raise them with the Iranian authorities." The arrests will place further strain on relations between London and Tehran. Iran and Britain are embroiled in a bitter row over Iran's nuclear ambitions, amid tit-for-tat expulsions of diplomats. Iran's foreign minister has warned Tehran is considering downgrading its ties with Britain. The country's intelligence minister claims some people with British passports were involved in post-election violence. State media say 20 people were killed in the protests.