Ukrainian President Petro Poroshenko said Thursday that Russia has moved troops and heavy artillery over the border into the territory of Ukraine and he has canceled his planned visit to Turkey.
Russia's Interfax news service said the Russian Defense Ministry denies Moscow sent military units to Ukraine.
At a meeting of the National Security and Defense Council, the Ukrainian president said, "Columns of heavy artillery, huge loads of arms and regular Russian servicemen came to the territory of Ukraine from Russia through the uncontrolled border area" to aid rebel forces.
Poroshenko said the situation is "extremely difficult" but "controlled enough for us" to remain level-headed, according to a statement from the president's office.
Poroshenko's remarks suggest that Ukraine now believes Moscow has mobilized troops directly into the country in addition to supplying weapons to pro-Russian armed groups in what could be a further escalation of tensions between the two countries.