The bodies of two Russian royals have been exhumed as the country's authorities re-open an investigation into the family's murder.

Tsar Nicholas II and his wife Alexandra were killed by Bolshevik revolutionaries in 1918, alongside their daughters Olga, Tatiana, Maria and Anastasia and their son, Alexei.

On Wednesday, investigators took samples from the remains of Nicholas and Alexandra in a bid to confirm the identification of Alexei's and Maria's remains.

While Nicholas, Alexandra and three of their daughters were found in a mass grave in the Urals in 1991 and identified by DNA testing seven years later, the remains thought to be those of Alexei and Maria were not found until 2007.

They were also found some distance away, one of many reasons why the Russian Orthodox Church doubts their identification and has asked they be re-examined ahead of being buried with the rest of the family.

Senior Investigator Vladimir Solovyov, who heads the investigating team, told Russia's TASS news agency: "The exhumation was done in the presence of representatives of the Russian Orthodox Church".

"The necessary samples were taken from the remains of Nicholas II and Empress Alexandra Fyodorovna".

"Also, samples for blood from the stains on the full-dress uniform of (Nicholas's grandfather) Emperor Alexander II, who was killed by radical revolutionaries on 1 March 1881".

The remains have now been returned to the crypt of the St Catherine sacrarium in St Peter and Paul's Cathedral in St Petersburg, officials said.

Nicholas, Alexandra and the three daughters found in 1991 were reburied in 1998 and canonised by the church in 2000.

It is thought Alexei and Maria will also be canonised before the centenary of the murders in 2018 but, until their remains are confirmed, they are being kept at the Russian State Archives.