Washington and Kiev say Moscow has amassed troops near Ukraine's borders and accuse Russia of planning an invasion.
US President Joe Biden said 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.
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 on Friday 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."
Russia invaded 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 Defence 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.
READ MORE: Why Ukraine matters to Russia so much
Guarantees of NATO's expansion
The Kremlin said Putin will seek binding guarantees precluding NATO’s expansion to Ukraine during a planned call with Biden, while the Ukrainian defense minister warned that Russia could invade his country next month.
Amid the mounting tensions, Putin’s foreign affairs adviser Yuri Ushakov told reporters Friday that arrangements have been made for a Putin-Biden call in the coming days, adding that the date will be announced after Moscow and Washington finalize details.
Ushakov noted that during the call with Biden, Putin will raise his demand for a legally-binding agreement that would “exclude any further NATO expansion eastward and the deployment of weapons systems that would threaten us on the territories of neighboring countries, including Ukraine.”
Russia has long pushed for such arrangements, Ushakov said, emphasizing that they have become particularly acute amid the latest buildup of tension. "It simply can’t continue like that,” he said.
The US has threatened the Kremlin with the toughest sanctions yet if it launches an attack, while Russia has warned that any presence of NATO troops and weapons on Ukrainian soil would cross a “red line.”