Ukrainian Official: Russian Tanks Have Entered the Country

The Russian Foreign Ministry told the BBC that the claim was "another fake piece of information."

According to Avakov, tanks crossed the border at a checkpoint controlled by pro-Russian separatists in the Luhansk region of eastern Ukraine. Armored vehicles and artillery were part of the columns, Avakov said, citing Ukrainian intelligence.

Two of the tanks split off and headed in the direction of Gorlovka, also known as Horlivka, and were attacked by Ukrainian military forces, Avakov said.

Part of that column was destroyed, he said.

CNN has not independently confirmed the incidents.

The countries have been engaged in a tense stand-off since March, when Russia annexed Crimea and massed troops along its border with Ukraine.

U.S. officials said in late May that thousands of Russian troops had withdrawn from the border. Thousands more who remained appeared to be preparing to depart, U.S. officials said at the time.

Western leaders have accused Moscow of fomenting instability in eastern Ukraine. Russian leaders say Ukraine has failed to move to implement a framework for peace worked out in Geneva, Switzerland, in April. They say Ukrainian leaders have carried out a campaign of violence against people living in the largely pro-Russian east.

Also on Thursday, Ukrainian President Petro Poroshenko and Russian President Vladimir Putin "held a substantial and long phone conversation," the Ukrainian President's media office said, but it was unclear if that conversation occurred before or after the reported tank incursion.

The leaders discussed Poroshenko's peace plan to resolve the situation in the east of Ukraine, the media office said.