The Ukrainian army said it had broken a rebel encirclement of Luhansk airport on Sunday night. A spokesman for the so-called Luhansk People's Republic said 30 volunteer fighters had been killed in Ukrainian fire on Oleksandrivka, a village to the east of the town, Russia's Interfax news agency said.

As military action continued on Monday near the rebel-controlled border town of Luhansk, Ukraine's defense minister said a Ukrainian AN-26 transport plane, taking part in the military campaign against the rebels, had been shot down by a rocket which was "probably" fired from Russian territory.

Officials said two crew members, out of the eight people on board, had been in contact with the army general staff and a search and rescue operation was underway. The fate of the other six people was not immediately known.

Defense Minister Valery Heletey said the plane had been flying at a height of 6,500 meters and was out of range of any weapon the separatists had.

"So the plane was downed from another, more powerful rocket weapon which was fired, probably, from the territory of the Russian Federation," he said, according to Poroshenko's website.

The rocket may have been a Pantsir ground-to-air or self-guided air-to-air rocket fired from a Russian plane, he said.

Lysenko said separatists, backed by what he described as Russian "mercenaries", had fired on Ukrainian border guards in an attempt to give cover as armored vehicles and equipment were being brought into the country. (rt/ez)