The US Secretary of State is expected to urge his Russian counterpart to ensure a 'transparent' investigation into the murder of an opponent of President Vladimir Putin. 

Boris Nemtsov is believed to have been working on a report into the presence of Russian troops in Ukraine before he was shot in Moscow on Friday.

Mr Nemtsov's allies have accused the Kremlin of being behind the murder - however, President Putin has condemned the killing as 'vile' and has promised to find those responsible.

Speaking to Reuters, former Russian Prime Minister Mikhail Kasyanov (pron: Mick-highl Kaz-yanoff) says he believes it's a turning point in the country's history.