Ukraine warned Friday that Russia could launch a large-scale attack on the country in late January as tensions soar over an alleged Russian troop build-up near Kiev's borders.

The United States and its allies have for weeks accused Russia of planning an invasion of Ukraine and massing troops along Kiev's borders.

Moscow seized Crimea from Ukraine in 2014 and has since backed separatists fighting Kiev in eastern Ukraine. The conflict has left more than 13,000 dead. Russia denies planning an attack. 

"The most likely time to reach readiness for escalation will be the end of January," Ukrainian Defence Minister Oleksiy Reznikov said.

Addressing parliament, Reznikov said Ukrainian intelligence was analysing all possible scenarios.

"The possibility of a large-scale escalation from Russia exists," he warned. 

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, in Moscow-controlled Crimea and in separatist-held areas in eastern Ukraine. 

The United States this week said it had "evidence" Russia could be planning an invasion, threatening Moscow with economic sanctions if it attacks. Russia has dismissed the allegations as "hysteria."

Reznikov said escalation was a "likely scenario". 

He said Ukraine and its allies should make the cost of a possible attack "unacceptable for the aggressor."

He reiterated that Ukraine was seeking a "political-diplomatic settlement" to the tensions and that it would not carry out "military provocations."

Ukraine and its Western allies accuse Russia of sending troops and arms across the border to support the separatists -- claims Moscow denies.