US vows to prevent any Russian invasion of Ukraine

US President Joe Biden said Friday he would make it "very, very difficult" for Russia to launch any invasion of Ukraine, which warned that a large-scale attack may be planned for next month.

Washington and Kiev say Moscow has massed troops near Ukraine's borders and accuse Russia of planning an invasion.

Biden and President Vladimir Putin are due to hold a video call shortly to discuss the rising tensions, both sides confirmed Friday.

Biden told reporters in Washington he was putting together "the most comprehensive and meaningful set of initiatives to make it very, very difficult for Mr. Putin to go ahead and do what people are worried he may do."

Moscow seized Crimea from Ukraine in 2014 and has since backed separatists fighting Kiev in the east of the country. The conflict has left more than 13,000 dead.

"The most likely time to reach readiness for escalation will be the end of January," Ukrainian Defense Minister Oleksiy Reznikov told parliament in Kiev on Friday.

The minister said a "winter training period" had started in Russia and that Moscow had already launched exercises near Ukrainian territory.

He estimated that Russia had around 100,000 troops near Ukraine's border. Russia denies any military build-up.

Kremlin foreign policy adviser Yuri Ushakov told reporters a date had been agreed for the Putin-Biden video summit but would not be announced until final details of the talks have also been set.

US Secretary of State Antony Blinken said the call was expected "in the very near future."