Russian President Vladimir Putin has ordered troops into two rebel-held regions in eastern Ukraine, after recognising them as independent states.
Russia said the troops have not yet been deployed but will be "peacekeeping" in the regions, which it has backed since 2014.
The US said calling them peacekeepers was "nonsense" and accused Russia of creating a pretext for war.
Several countries have announced sanctions in response.
Ukraine's president said his country was "not afraid of anything or anyone".
In a late-night televised address to the nation, President Volodymyr Zelensky called for "clear and effective actions of support" from Ukraine's international allies.
Fears over an invasion have been rising in recent months, as Russia has put some 150,000 troops along Ukraine's borders, according to US estimates.
Russia has been backing a bloody armed rebellion in eastern Ukraine for the past eight years. Some 14,000 people – including many civilians – have died in fighting since then.
[Source: BBC]
