A former separatist commander believed to be linked to the downing of Malaysia Airlines flight 17 has been captured in eastern Ukraine and smuggled across the frontlines to government-controlled territory.

Vladimir Tsemakh, who turned 58 in confinement in Kiev on Thursday, was captured by Ukrainian agents in the Russia-backed splinter state of the Donetsk People's Republic on June 27, relatives told the BBC Russian service.

He formerly commanded anti-aircraft defences in Snizhne, which according to an international investigation is near to where the Russian missile that brought down MH17 was fired. All 298 people on board were killed when the Boeing 777 was blown out of the sky on 17 July 2014, including 10 Britons.

Ukrainian media have reported that Mr Tsemakh could be a “valuable witness” in the international investigation of the tragedy, while the authorities have declined to comment on the incident. Mr Tsemakh's relatives have claimed that Kiev plans to “frame” him for the disaster.

Last month, Dutch investigators brought murder charges against three Russian and one Ukrainian separatist fighter for delivering a Buk missile from the Russian border to the site of the shoot down. They also called for information on Russian troops suspected of involvement.