Ukraine has accused Russia 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 on Thursday, but the Reuters news agency reported that three of its correspondents had seen the tanks in the border town of Snizhnye.
"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," Arseny Avakov, Ukraine's interior minister, 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, with Russian President Vladimir Putin promising to tighten controls at the border.
In a separate incident, a minibus belonging to Denis Pushilin, the separatist leader of Donetsk, was blown up in the eastern city, killing two of his entourage and wounding two others, the rebel government said.
The explosion late on Thursday took place outside the headquarters of the separatist movement in the former regional government building in the heart of the coal mining city.
"Pushilin was not in the vehicle," the rebel authorities said.
Pushilin survived another apparent assassination attempt on Saturday when his assistant was shot dead in central Donetsk.
'Ready for truce'
Sergey Lavrov, Russia's foreign minister, said on Thursday that the pro-Moscow separatist rebels in eastern Ukraine are ready for a ceasefire, but that Kiev has to initiate the process.
Lavrov was also quoted by Russian news agencies as saying Moscow will introduce a resolution in the United Nations on the Ukrainian crisis, but that Russia was not seeking authorisation to send in peacekeeping troops.
He said the proposed UN resolution would concentrate on demanding fulfilment of proposals set forth in a so-called "road map" for resolving the conflict.
The road map was drawn up by the Organisation for Security and Cooperation in Europe in May to give impetus to a deal reached in Geneva by the European Union, Russia, Ukraine and the United States to try to end the crisis in Ukraine.