Satellite images show a 40-mile long convoy of Russian military vehicles near the Ukrainian capital.
Airstrikes have also been carried out on an airbase on the edge of Kyiv.
The first round of talks between the two sides have ended without agreement.
Meanwhile, the UN says more than half a million people have now fled Ukraine into neighbouring countries.
Communications manager for Ukrainian football team Shakhtar Donetsk, Yuriy Sviridov, says some of its players have stayed to fight.
"Some of them in Kyiv, some of them in other cities. They came without their families, it's getting more and more unsafe to be in Kyiv."
