Ukraine’s President Volodymyr Zelensky has warned that Russia could start “a major war in Europe” any day now, and called on Russians to oppose it.
In a late-night address, he said he had unsuccessfully sought talks with Russian President Vladimir Putin.
“I initiated a telephone call with the president of the Russian federation. Result: silence,” Mr Zelensky said.
He said Russia had almost 200,000 troops and thousands of combat vehicles on Ukraine’s borders.
And switching from Ukrainian into Russian, Mr Zelensky made an emotional appeal to Russians to reject an attack, saying they were being lied to about Ukraine.
“Who can stop (the war)? People. These people are among you, I am sure,” he said.
Ukraine’s leader said his country was ready for a Russian attack, vowing: “If they [Russia] attack, if they try to take our county – our freedom, our lives, the lives of our children – we will be defending ourselves.”
“As you attack, it will be our faces you see, not our backs,” the Ukrainian president said.
It came as Russian troops, ordered into Ukraine’s two rebel-held regions, were reportedly edging closer to the border.
Moscow said the breakaway regions in eastern Ukraine had asked for military support.
Airspace restricted over Ukraine
Early on Thursday, Ukraine said it had restricted civilian flights in its airspace due to “potential hazard”. The move followed warnings from the conflict zone monitor Safe Airspace, which said planes risked being shot down unintentionally or affected by cyber attacks.
It was not initially clear if the notice amounted to an outright ban on civilian flights. It is due to expire at 2359 GMT on Thursday, unless extended further.
Russia has also issued a civil aviation notice warning that it is closing virtually all of eastern Ukrainian airspace on the border with Russia, according to a monitoring group.
The Open Source Intelligence Monitor says no “limit or ending time” has been provided so far by Moscow.
The United Nations Security Council is holding an emergency session from 02:30 GMT to discuss the deteriorating situation.
Ukraine has declared a state of emergency and told millions of its citizens living in Russia to return home. The 30-day emergency status allows authorities to adopt heightened security measures, including curfews.