German prosecutors are investigating whether two Russian dissidents living in Berlin have been poisoned.

  Earlier this month, the German weekly Focus reported that doctors had detected high levels of mercury in the blood of Viktor and Marina Kalashnikov.

  He was a former colonel in the KGB while she is a historian and both have been critical of the Kremlin.

  In 2006, former Russian agent Alexander Litvinenko was murdered by radioactive poisoning in London.

  British investigators suspect former KGB agent Andrei Lugovoi is behind his murder.

  A spokesman for the public prosecutors' office in Berlin confirmed that an investigation had been opened into whether the Kalashnikovs have been deliberately poisoned.

  "It is being carried out by a department dealing with politically motivated crimes," the spokesman told AFP news agency.

  Health problems

  Viktor Kalashnikov is reported to have worked for the KGB in Brussels and Vienna before joining Boris Yeltsin's research staff.

  Since the late 1990s both he and his wife have worked as freelance journalists and commentators.

  In recent years they have been moving around Europe, including Ukraine, Poland and Estonia.

  The Times reported that they started experiencing strange health problems - their skin burnt and they would be seized by bouts of restlessness, blinding headaches and sudden pains in the spine.

  In October this year, in a Berlin hotel room "we went sort of crazy," Marina Kalashnikov told the newspaper, "wandering feverishly around all night, unable to concentrate".

  The couple then had tests at Berlin's Charite hospital. These reportedly revealed 53.7 microgrammes of mercury per litre in Viktor's blood, and 56 in Marina's.

  The usual level is around one to three microgrammes per litre.

  Mr Kalashnikov has lost a lot of weight in recent months and medical experts have reportedly recommended that the couple undergo further tests and be watched closely.