Russian President Vladimir Putin has ordered troops into two rebel-held regions in eastern Ukraine, after recognising them as independent states.
Reportedly, the footage released overnight appeared to show Russian military vehicles heading towards the Ukrainian border but Russia said the troops would be "peacekeeping" in the breakaway regions, which it has backed since 2014.
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But the US said calling them peacekeepers was "nonsense" and accused Russia of creating a pretext for war.
However, the Ukraine’s president Volodymyr Zelensky said his country was "not afraid of anything or anyone" but called for "clear and effective actions of support" from Ukraine’s international allies.
"It is very important to see now who our real friend and partner is, and who will continue to scare the Russian Federation with words only," he added.
Several countries, including the UK, are considering introducing new sanctions against Russia in response to the move.
Fears of invasion have been rising in recent months, as Russia has massed some 150,000 troops along Ukraine’s borders, according to US estimates.
Russia has been backing the armed rebellion in eastern Ukraine for the past eight years. Some 14,000 people – including many civilians – have died in fighting since then.
In an hour-long address on Monday, Mr Putin said Ukraine was an integral part of his country’s history, and described eastern Ukraine as "ancient Russian lands".
