The charge, dismissed by Moscow, dealt a blow to already slim hopes of progress at talks on Tuesday towards ending the conflict between pro-Russian rebels and Ukrainian troops, in which more than 2,000 people have been killed.
Ukrainian President Petro Poroshenko, who is due to meet Russia's Vladimir Putin in the Belarussian capital of Minsk, expressed "extraordinary concern" at the Russian move, his press service said, setting the scene for a possibly angry encounter.
The Ukrainian military said a group of Russian forces, in the guise of separatist rebels, had crossed into south-east Ukraine with 10 tanks and two armored infantry vehicles. It said border guards had halted the column outside Novoazovsk, Ukraine's most south-easterly point on the Azov Sea, Reuters reported.
"This morning there was an attempt by the Russian military in the guise of Donbass fighters to open a new area of military confrontation in the southern Donetsk region," military spokesman Andriy Lysenko told journalists.
Donbass is the name given to the industrialized and mainly Russian-speaking east of Ukraine, where two regions -- Donetsk and Luhansk -- have declared independence from Ukraine in an attempt to join Russia.
Lysenko later added that two tanks in the column had been destroyed and several members of "an intelligence-sabotage group" had been seized.
"The area is now blocked by Ukrainian troops," he said.
Breackthrough unlikely with the rebels largely encircled in the cities of Donetsk and Luhansk, Ukraine has been pressing its advance while its Western allies have waited nervously to see if Moscow will intervene to prevent the separatists being crushed.