Russian President Vladimir Putin has ordered troops into two rebel-held regions in eastern Ukraine, after recognizing 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”.
“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 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.
At an emergency meeting of the United Nations Security Council, US Ambassador Linda Thomas-Greenfield dismissed Russia’s claims that troops would be taking on a “peacekeeping” role, saying: “We know what they really are.”
Recognizing Luhansk and Donetsk as independent was part of Russia’s bid to create a reason to invade Ukraine, she said.
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.
In recent years, Russian passports have been given out to large numbers of people in Donetsk and Luhansk.
The rebel-held areas have been evacuating women, children, and the elderly to Russia since late last week.
In an hour-long address on Monday, Putin said Ukraine was an integral part of his country’s history and described eastern Ukraine as “ancient Russian lands”.
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