The Security Service of Ukraine (SBU) has indisputable evidence of Russia's involvement in an attack on a Ukrainian An-26 military transport aircraft in Luhansk region on July 14, SBU Chief Valentyn Nalyvaichenko has said.

"There is incontrovertible evidence," he told reporters in Kyiv on Tuesday, when asked whether the SBU has evidence that Russia was involved in the attack on the Ukrainian An-26 on Monday.

"The president has given instructions, and by the end of the day the investigation will be completed. Incontrovertible evidence, with maps, with all sectors, will be prepared," Nalyvaichenko said.

He added that this evidence would be provided to President Petro Poroshenko and made public.

As reported, a Ukrainian An-26 military transport aircraft was shot down near Luhansk on July 14. There were seven crew members and one person accompanying the cargo on board the plane. They all bailed out of the plane.

Ukrainian Defense Minister Valery Heletei had told Poroshenko that the Antonov An-26 was at an altitude of about 6,500 meters when it was hit and would have been out of range of portable surface-to-air missile systems.

That meant the plane "was hit with another, more powerful weapons system, which was probably used from the territory of the Russian Federation," Heletei said.

The chief spokesman for Ukraine's National Security and Defense Council expressed suspicion that the aircraft had been attacked by a Russian plane.

"Two possibilities are under consideration on the basis of a report from the Ukrainian pilots: either a strike of a ground-based missile fired from a Pantsir missile system or a strike by an … air-to-air homing missile that was fired from a Russian aircraft that had scrambled from the Russian airport at Millerovo," Andriy Lysenko told a briefing.