Poroshenko called a meeting of Ukraine's national security council as the leader of the main separatist group in southeastern Ukraine told Russian television that up to 4,000 Russians are fighting against Ukrainian military.

"There are active soldiers fighting among us who preferred to spend their vacation not on the beach, but with us, among their brothers, who are fighting for their freedom," Aleksandr Zakharchenko said.

The Washington Post reports two Russian armored columns captured the key southeastern coastal town of Novoazovsk on the Sea of Azov near the Russian border after the Ukrainian military retreated.

The claim was issued by the Kiev government's military information service.