As Ukraine’s soldiers continue to battle separatists in the east, Kyiv claims Russia has let rebels take tanks across its border into Ukrainian territory to fight government forces.
At least three tanks are said to have made the journey, with a column entering in the Luhansk area before moving into the neighbouring Donetsk region.
Moscow has always denied providing military support to the rebels.
“According to intelligence and surveillance, despite the Russian Federation’s statements that it welcomes the peace process and that the order has been given to strengthen border controls, over the last three days we have observed columns passing through checkpoints captured by terrorists,” Ukraine’s Interior Minister Arsen Avakov told reporters.
In a telephone call, Ukraine’s new President Petro Poroshenko told Russian leader Vladimir Putin that such actions were “unacceptable”.
Separately, up to two people were killed in an apparent assassination attempt on the self-declared leader of the rebel Donetsk People’s Republic. Denis Pushilin was not in his vehicle when it blew up in the city centre. He had already escaped an attack earlier this month.
It comes as Russia accused Ukraine of using banned incendiary bombs in its military operations – something Kyiv denies.