By Natalia Zinets and Richard Balmforth

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.

The 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.

"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 (Ukrainian army) checkpoints 29 and 31," he said, pinpointing an area near the town of Slovyanoserbsk, northwest of the city 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 motorised infantry had been tracked inside Russia heading towards the Ukrainian border, as well as an artillery division, he said.

Lysenko had no immediate figures for estimated casualties.

Ukraine's assertion that its troops were now engaged with Russian regular forces in its separatist-minded eastern regions followed a flurry of charges by Kiev that the Russian military was stepping up incursions to support the rebels.

Despite what the West and Kiev say is incontrovertible proof, Moscow has consistently denied that any of its regular forces are deployed in Ukraine.

The region around Luhansk includes a huge stretch of border with Russia and, with large parts of it under the control of separatists, is vulnerable to an inflow of military equipment and Russian forces.

Prime Minister Arseny Yatseniuk said on Monday that Russian-made rocket and weapons systems were pouring into the country and Lysenko on Tuesday said two battalion tactical groups, each of about 400 men, had crossed into Ukraine from Russia - a charge dismissed as "absolute nonsense" by the Russian defence ministry.

In a separate statement, the Ukrainian defence ministry said Ukrainian forces had withdrawn from one of the two checkpoints under attack and, with the aid of reinforcements, were trying to dislodge enemy forces.

FOUR-WAY SUMMIT DUE

Ukraine's announcement was certain to ratchet up tension between Kiev and Moscow on the eve of a planned meeting in Berlin of four foreign ministers that should bring Ukrainian Foreign Minister Pavlo Klimkin face to face with Russia's Sergei Lavrov.

Wednesday's meeting, which also brings in the German and French foreign ministers, has been called to give fresh impetus to stalled efforts to end the worst crisis between Russia and the West since the end of the Cold War.

The crisis started with the popular overthrow of a Moscow-backed president by street protests in Kiev a year ago.

Russia subsequently annexed Ukraine's Crimea peninsula and threw its weight behind a pro-Russian separatist rebellion, leading to a conflict in which more than 4,800 people have been killed and the West has applied sanctions against Russia.

Ukrainian forces at the weekend launched a counter-offensive to reclaim ground lost to separatists near the international airport in the big city of Donetsk, bringing condemnation from Moscow, which said this had damaged the prospects for a four-power summit on the conflict.

Earlier on Tuesday, Ukraine's Klimkin pressed for further meetings to implement agreements reached in Minsk, Belarus, last September with Russia and separatist leaders including strengthening a much-violated ceasefire.

Klimkin said separatist forces had abused the Minsk deal by seizing 500 sq km (194 sq miles) of territory beyond agreed separation lines since it was struck.

(Additional reporting by Alessandra Prentice; Editing by Hugh Lawson)