Concerns have been raised about the investigation into the Malaysia Airlines disaster after international observers said armed rebels were restricting their access to the crash site.
Europe security monitors said they had gained partial access to the site in eastern Ukraine but were stopped from going further “for their own” safety.
Six air accident investigators from Britain and a Malaysian disaster response team are due in the capital Kiev later.
Some 298 people, including 10 Britons, nearly 200 Dutch passengers and 80 children, were killed when flight MH17 was brought down near Grabovo, Donetsk.
The British Government has joined the United States in blaming pro-Russian separatists for the catastrophe.
Downing Street said it appeared “increasingly likely” a surface-to-air missile had been fired from near Torez, in territory controlled by rebels seeking closer ties to Moscow.
The Boeing 777-200 was flying on an established route from Amsterdam to Kuala Lumpur that had been declared safe by the International Civil Aviation Organisation.
It came down close to where Ukrainian forces have been battling separatists, with both sides blaming each other for the disaster.
Ukraine condemned the “act of terrorism” and released what it claimed was a recording of an intercepted phone call between two Russian military intelligence officers, discussing the downing of the plane.
In an unverified video, apparently taken moments after the plane was hit, an alleged pro-Russian rebel boasts: “Wow, that was a blast.
However, Russia pointed the finger at its neighbour, saying it picked up radar activity from a Ukrainian Buk missile system south of Donetsk when the aircraft came down.
Around a hundred of those killed were delegates on their way to an international conference on Aids in Melbourne, Australia, including world-renowned researcher Joep Lange.