Russia has extended military drills near Ukraine's northern borders.
It comes amid fears that two days of shelling along the contact line between soldiers and Russia-backed separatists in eastern Ukraine could spark an invasion.
The exercises, which were initially set to finish on Sunday, brought a sizeable contingent of Russian forces to neighbouring Belarus, which borders Ukraine to the north.
The presence of the Russian troops increased fears that they could be used to sweep down on the Ukrainian capital Kyiv.
The announcement came from the defence minister of Belarus, who said the two countries would "continue testing the response forces."
Western leaders have warned that Russia looks poised to attack its neighbour, which is surrounded on three sides by about 150,000 Russian soldiers, warplanes and equipment.
Watch: The UK troops deploying in response to Russia's military build-up.
Russia held nuclear drills on Saturday as well as the conventional exercises in Belarus and has ongoing naval drills off the coast in the Black Sea.
Ukrainian president Volodymyr Zelenskyy called on Russian President Vladimir Putin to choose a place where the two leaders could meet to try to resolve the crisis. Russia has denied plans to invade.
"Ukraine will continue to follow only the diplomatic path for the sake of a peaceful settlement," Zelenskyy said on Saturday at an international security conference in Munich, Germany. There was no immediate response from the Kremlin.
A Russian invasion of Ukraine could lead to the largest conflict in Europe since World War Two, Boris Johnson has warned.
The Prime Minister said he wants people to "understand the sheer cost in human life" that an incursion into Ukraine would bring, with casualties on both the Russian and Ukrainian sides, as he continued to urge Moscow to engage in peace talks.
He also admitted hard-hitting financial sanctions may not be enough to prevent Russian President Vladimir Putin from signing off on an invasion of Ukraine.