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 armored personnel carriers, other armored 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 Kyiv.
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 Kyiv 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."
Ukraine's president signaled readiness to hold talks
On Wednesday, Ukraine's new president signaled he would be ready to hold talks with opponents in eastern Ukraine if pro-Russian separatists waging an insurgency there agreed to lay down their weapons.
The rebels show no sign of giving up their arms, but opening talks would be a big step on the road to peace, building on Ukrainian President Petro Poroshenko's meetings this week with Moscow's envoy and with Russian President Vladimir Putin in France last week.
Poroshenko, trying to act swiftly after being sworn in as president on Saturday, was quoted by his press office as telling the governor of the Donetsk region of east Ukraine that he would not rule out holding "roundtable" talks with "different parties."
"We do not need negotiations for the sake of negotiations. Our peace plan must become the basis for further de-escalation of the conflict," a statement on the president's website quoted Poroshenko as saying at a meeting with Governor Serhiy Taruta.
He added: "Terrorists must lay down their weapons."
This footage, posted on Ukrainskaia Pravda's website, purportedly shows one of the tanks in question. The authenticity of the video was not verified.