KIEV (Reuters) - Ukrainian army units came under attack from Russian regular forces in eastern Ukraine on Tuesday and heavy fighting was taking place, a Ukrainian military spokesman said.
"In spite of preliminary agreements, Ukrainian military units were attacked in the north of the anti-terrorist operational zone by regular military formations of the armed forces of Russia," the spokesman, Andriy Lysenko, said.
"Heavy fighting is continuing near the (Ukrainian army) checkpoints 29 and 31," he said, pinpointing an area near the town of Slovyanoserbsk, northwest of Luhansk.
"Ukrainian forces have stopped the advance of Russian troops ... The situation in the conflict zone is serious but under our control," Lysenko said at a news briefing.
Three more battalion tactical groups of Russian motorized infantry had been tracked inside Russia heading towards the Ukrainian border, as well as an artillery division, he said.
Lysenko's announcement at a specially called news briefing was one of the boldest assertions yet by Ukraine of direct Russian military involvement in the nine-month conflict between pro-Russian separatists and government forces.
Despite what the West and Kiev says is incontrovertible proof, Moscow has continued to deny that any of its regular forces are deployed in Ukraine.
(Reporting by Natalia Zinets; Writing By Richard Balmforth; Editing by Alessandra Prentice and Mark Trevelyan)