MH17 investigators release intercepted phone calls showing Ukraine rebels' ties to Russia

Investigators looking into the downing of flight MH17 over Ukraine released intercepted phone calls on Thursday showing close links between "high-ranking" Russian officials and rebel suspects facing trial over the crash.

The international probe said the communications raised questions about the possible involvement of top officials in Moscow in the deployment of the missile that shot down the Malaysia Airlines plane in 2014.

The intercepts also showed that ties between the Kremlin and fighters from the separatist Donetsk People's Republic (DPR) in eastern Ukraine were "much closer" than originally believed, the probe said.

"There was almost daily telephone contact between the leadership of the DPR and their contacts in the Russian Federation," the Dutch-led Joint Investigation Team said in a statement.

"They spoke with leaders in Moscow, near the border with Ukraine and in Crimea. Communication mostly took place via secure telephones provided by the Russian security service."

The investigation team is appealing for more witnesses ahead of the trial in the Netherlands next year of three Russian nationals and one Ukrainian over the downing of the jet.

The passenger plane travelling from Amsterdam to Kuala Lumpur was shot down by a Russian-made BUK missile in 2014 over rebel-held eastern Ukraine, killing all 298 people on board.

Investigators have previously said the missile was transported to Ukraine from a Russian army anti-aircraft brigade based in the Russian city of Kursk.

- 'Mandate' from minister -

The suspects facing trial -- former Russian intelligence agent Igor Girkin, also known by his pseudonym "Strelkov", Sergei Dubinsky, Oleg Pulatov, Leonid Kharchenko -- were all leading members of the DPR.

They are all expected to be tried in absentia.

A number of the calls involved Vladislav Surkov, a key aide to Russian President Vladimir Putin, the investigators said. One of them involves him promising military reinforcements from Russia in 2014.

In another call, from 2015, he speaks to defendant Kharchenko over concerns that fellow suspect Dubinskiy will be arrested for illegal crossing over into eastern Ukraine.

Russian Defence Minister Sergei Shoigu is meanwhile mentioned in a number of telephone calls between rebels, with one of them saying that fighters were coming "with a mandate from Shoigu."

"Ties between Russian officials and DPR leaders appear to have been much closer" than originally believed, Andy Kraag, the head of Dutch police's Criminal Investigations Division, said in a video statement.

"The indications of close ties between leaders of the DPR and Russian government officials raise questions about their possible involvement in the deployment of the BUK... which brought down flight MH17," the statement by investigators added.

Investigators said they specifically wanted to know if Surkov and Shoigu "have a role in the planning and execution of military operations in the summer of 2014 in eastern Ukraine" when the flight was downed, it added.

They appealed for more witnesses to come forward.

The trial is due to start in the Netherlands on March 9 next year.

The investigation team involves Australia, Belgium, Malaysia, Ukraine and the Netherlands, the countries hardest hit by the disaster. 196 of the victims were Dutch.