The British government`s cultural arm, caught in a political row between London and Moscow, said on Thursday its Russian regional offices had halted work because local staff were being interviewed by Russia`s security service. The British Council has been involved in an escalating dispute with Moscow over the legality of its Russian branches, part of a wider diplomatic argument over the murder in 2006 of a former Russian agent in Britain. The Council said it had halted work at its office in the Urals city of Yekaterinburg because Russian staff were being interviewed by the Federal Security Service (FSB), the main successor to the Soviet-era KGB. It closed another office in the northern city of St Petersburg on Wednesday after FSB interviewed its staff. The office remained closed on Thursday. "The normal services usually offered by the British Council in Yekaterinburg can not be offered today because FSB interviews with staff are continuing," a spokeswoman said by telephone from London. "The staff there are not working today." A spokesman for British Prime Minister Gordon Brown warned Russia on Thursday against intimidating staff. "Any attempts to intimidate staff is in our view completely unacceptable," Brown`s official spokesman said. A Reuters correspondent in Yekaterinburg said the British Council`s plaque had been taken down from the wall of the local consulate building. A Reuters photographer in St Petersburg said the British Council was still closed. Britain and Russia, linked trade and investment worth billions of dollars a year, have been trading barbs since the 2006 murder of Alexander Litvinenko, who was poisoned in London with a rare radioactive isotope. Each country expelled four diplomats in the spat after Moscow refused to extradite Russian businessman Andrei Lugovoy to face trial for the murder. He denies any role in the murder. British Foreign Secretary David Miliband warned Russia on Wednesday against intimidating British Council staff after the FSB held interviews with local staff. The FSB said they invited Russian employees for interviews to tell staff that they might be used by Britain "in provocative games." The U.S. Department of State said it regretted Russian actions against British Council staff. "We regret reports of an escalation of actions taken by Russian authorities against the staff and activities of the British Council in St Petersburg and Yekaterinburg," State Department spokesman Sean McCormack said. "We hope this matter will be resolved fairly and expeditiously and in a manner that will allow the British Council to continue its good work in Russia."