The United Nations Security Council has scheduled an emergency meeting for August 28 on the invasion of Ukraine by Russian forces.
The meeting comes after Ukraine's President Petro Poroshenko said Russia had deployed forces in eastern Ukraine.
Meanwhile, British Prime Minister David Cameron has warned of "further consequences" for Moscow unless it halts the advance of Russian tanks into Ukraine.
A top NATO official said on August 28 that more than 1,000 Russian soldiers have invaded eastern Ukraine, with 20,000 more mobilized just across the border.
Kyiv's National Security and Defense Council says Russian tanks and troops seized the Ukrainian border town of Novoazovsk on the Sea of Azov, and that others were taking positions in the east.
A Ukrainian military spokesmen said government forces have regrouped to defend the city of Mariupol to the west of Novoazovsk.