In a meeting with his Cabinet Thursday evening, Poroshenko directed them to establish the State commission and to consider engaging experts from the International Civil Aviation Organization as well as representatives from the Netherlands and Malaysia.
The president's office acknowledged the possibility that the plane had been shot down.
If MH17 was targeted, it would be the third aircraft to be shot down this week over eastern Ukraine.
On Wednesday evening, a Ukrainian SU-25 fighter plane was shot down, and Ukrainian officials claim Russia is responsible. "Russia carried out another provocation on July 16 at around 1900," National Security and Defense Council spokesman Andriy Lysenko said Thursday. "A military aircraft of the Russian Armed Forces launched a missile strike against a Su-25 aircraft of the Ukrainian Armed Forces, which was performing tasks on Ukrainian territory." The pilot safely ejected.
A Ukrainian military transport plane was shot down Monday by a "more powerful missile that was probably launched from Russian territory," Defense Minister Valery Heletey said in a report submitted to the president.
The president's office wrote in a statement that if MH17 was shot down, it was not targeted by the Ukrainian military, adding that "We are confident those responsible for the given tragedy will be brought to justice."
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