Kiev - Ukrainian President Petro Poroshenko was cutting short a foreign trip on Wednesday after Kiev accused Russian regular forces of attacking its troops in the country's east.The accusation against Moscow on Tuesday was one of the boldest assertions yet by Ukraine of direct Russian military involvement since the conflict between pro-Russian separatists and government forces began more than nine months ago."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 spokesperson, Andriy Lysenko, said.Roman Turovets, a spokesperson for the military operation in the east, said by telephone: "Ukrainian troops have stopped the advance of the enemy [and] are regrouping to push the enemy back."Tuesday's clashes broke out at two checkpoints near the town of Slovyanoserbsk, northwest of the city of Luhansk.Russian motorised infantry had been tracked inside Russia heading towards the Ukrainian border, as well as an artillery division, Lysenko said. There were no casualty figures.The new fighting prompted Poroshenko to decide to cut short his visit to the World Economic Forum in the Swiss resort of Davos and return to Ukraine, his spokesperson said on Facebook.Moscow deniesUkraine's assertion that its troops were now engaged with Russian regular forces in its separatist-minded eastern regions followed charges by Kiev that the Russian military was stepping up incursions to support the rebels.Despite what the West and Kiev say is clear proof, Moscow denies any of its regular forces are deployed in Ukraine.The region around Luhansk includes a big stretch of border with Russia and, with large parts under the control of separatists, it is vulnerable to an inflow of military equipment and Russian forces.Prime Minister Arseny Yatseniuk said on Monday Russian-made rocket and weapons systems were pouring into the country. On Tuesday, Lysenko said two battalion tactical groups, each of about 400 men, had crossed into Ukraine from Russia - a charge dismissed as "absolute nonsense" by Russia's Defence Ministry.In a separate statement, the Ukrainian Defence Ministry said Ukrainian forces had withdrawn from one of the two checkpoints that were under attack and were trying to dislodge enemy forces.Four-way summitUkraine's announcement was certain to ratchet up tension between Kiev and Moscow before a 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 involves the German and French foreign ministers, is part of attempts to end the worst crisis between Russia and the West since 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 backed the 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 city of Donetsk. Moscow said this had damaged the prospects for a four-power summit.