Russian President Vladimir Putin has ordered troops into two rebel-held regions in eastern Ukraine, after recognising them as independent states.
Footage released overnight appeared to show Russian military vehicles heading towards the Ukrainian border.
Russia said the troops would be “peacekeeping” in the breakaway regions, which it has backed since 2014.
But the US said calling them peacekeepers was “nonsense”.
It accused Russia of creating a pretext for war.
Ukraine’s president said his country was “not afraid of anything or anyone”.
According to a BBC report, several countries, including the UK, are expected to introduce sanctions against Russia in response to the move.
Fears over an invasion have been rising in recent months, as Russia has massed some 150,000 troops along Ukraine’s borders, according to US estimates.
