Western countries spoke up at the UN in Geneva Wednesday to declare "unwavering" support for Ukraine's sovereignty, independence and territorial integrity, amid growing concern Russia will invade.

As the United Nations Human Rights Council gathered to hear an update on the rights situation across Ukraine, diplomats from the US, EU, Britain and elsewhere voiced alarm at signs of looming Russian aggression against the country.

"We are gravely concerned by evidence that Russia has developed plans to take further aggressive action against Ukraine," Patrick Elliot, a human rights officer at the US mission in Geneva, told the council.

"We are unwavering in our support of Ukraine's sovereignty, independence and territorial integrity." 

British Ambassador Simon Manley also voiced alarm at the "pattern of destabilising activity and military buildups on Ukraine's border", warning that "Russia's activity poses a threat to Ukraine's and international security."

Ukraine and its Western allies have accused Moscow of massing tens of thousands of troops near its border with Ukraine in preparation for an invasion. 

Kiev's military command believes a possible escalation could come at the end of January. 

Moscow has said the troop build-up is part of routine deployments, saying it has the right to do what it wants on its territory.

Ukrainian Ambassador Yevheniia Filipenko warned the rights council Wednesday that "the impact of possible invasion will have disastrous consequences for the security, humanitarian and human rights situation in my country and well beyond."

The comments came after UN deputy rights chief Nada Al-Nashif presented the council with an update on the rights situation across Ukraine.

She did not touch on the feared Russian buildup at the border.

But she did decry significant restrictions of the freedom of expression and assembly in the Crimean peninsula, which was annexed by Russia in 2014, and in eastern regions where Kiev has been fighting a pro-Russian insurgency since then.

Al-Nashif also highlighted rights concerns in areas under the control of the Ukrainian government, including the "lack of accountability" for threats and violence targeting rights defenders, media workers and women and gay rights activists, among others.

Russian representative Yaroslave Eremin seized onto that criticism, slamming Kiev for "maintaining an atmosphere of impunity" exploited by the far right. 

He also urged UN rights office to "stop including in their reports (on Ukraine), matters related to Russian regions" -- referring to Crimea.