Ukraine said Monday that a transport plane shot down in the eastern part of the country was “probably” fired on from Russia, as the Ukrainian government continued to assert that Russian troops are fighting alongside pro-Russia separatists against government forces.

Col. Gen. Valery Heletey, Ukraine’s defense minister, said the Ukrainian AN-26 transport plane, was flying at more than 21,000 feet, meaning it was at an altitude where it couldn’t be threatened by the type of missiles used by the separatists.

"Thus, the plane was downed by another, more powerful missile weapon that probably was used from the territory of the Russian Federation,” Heletey said in a statement posted on the website of Ukrainian President Petro Poroshenko.

The government didn’t say anything about casualties, but military spokesman Vladyslav Seleznyov told Reuters that he couldn’t confirm that everyone on board, believed to be two people, were alive. A search and rescue mission for the plane was underway near Luhansk, a city on the Ukrainian-Russian border where there has been intense fighting between pro-Russia separatists and the Ukrainian army for three months.

The attack on the transport came as Kiev accused Moscow of having its military officers aid pro-Russia separatists in Ukraine.

"Information has ... been confirmed that Russian staff officers are taking part in military operations against Ukrainian forces,” Poroshenko said Monday, according to Reuters. His allegations followed similar claims made Sunday to the European Union, hoping the body would punish Russia with further sanctions.

Meanwhile, Russia accused Ukraine of killing a Russian man with a shell that was fired across the border.