Ukraine's interior minister accused Russia on Thursday of allowing three tanks and other military vehicles to cross the border into east Ukraine to help pro-Russian separatists there.
Russia did not immediately respond to the accusations but Reuters correspondents saw three tanks in the border town of Snizhnye in east Ukraine.
"We have observed columns passing with armoured personnel carriers, other armoured vehicles and artillery pieces, and tanks which, according to our information, came across the border and this morning were in Snizhnye," Ukrainian Interior Minister Arseny Avakov told reporters in Kiev.
He said Ukrainian forces had destroyed part of the column and fighting was still under way, but gave no further details.
Russia has denied aiding the separatists, who have taken control of several towns and cities in mainly Russian-speaking east Ukraine, and Russian President Vladimir Putin has promised to tighten controls at the border.
Avakov said the Ukrainian intelligence reports, however, undermined confidence in Russia's commitment to efforts to end weeks of violence in the east, where rebels who oppose the pro-Western leadership in Kiev control several towns and cities.
He said the columns had come across the border at checkpoints seized by the rebels "despite the Russian Federation's statements that it welcomes the peace process and that the order has been given to strengthen border controls."
Ukrainian President Petro Poroshenko told Putin over the phone that the tank's border crossing was "unacceptable," according to a presidential spokesman.
—By Reuters, with CNBC