A convoy of “up to 100” tanks, armoured vehicles and rocket launchers was seen travelling on a road toward Telmanove, a town about 80 kilometres south of rebel stronghold Donetsk and 20 kilometres from the Russian border, Ukraine’s army said in a statement.
The army did not give details about the personnel on board the vehicles or when the column was thought to have entered Ukraine.
A military source said the convoy had come from Russia.
“We believe that this is Russian equipment. You cannot buy 100 tanks at a market in Donetsk or Lugansk,” the source said.
“Of course they have been moved from across the border.”
If it turns out Russian soldiers are fighting in Ukraine it could further deepen the country’s crisis — the focus of talks this week between Russian president Vladimir Putin and Ukrainian prime minister Arseny Yatseniuk.
US ambassador to Ukraine Geoffrey Pyatt said in a Twitter post: “The new columns of Russian tanks and armour crossing into Ukraine indicates a Russian-directed counter-offensive may be underway.”