The United States has warned the United Nations it has information that Russia has lists of Ukrainians "to be killed or sent to camps" in the event of an invasion, according to a letter sent to the U.N. rights chief and obtained by Agence France-Presse (AFP) Sunday.

The letter, which came as Washington warned of an imminent invasion by Russian troops massed near the Ukrainian border, said the U.S. is "deeply concerned" and warns of a potential "human rights catastrophe."

The U.S. has "credible information that indicates Russian forces are creating lists of identified Ukrainians to be killed or sent to camps following a military occupation," the letter said.

"We also have credible information that Russian forces will likely use lethal measures to disperse peaceful protests or otherwise counter peaceful exercises of perceived resistance from civilian populations," said the message, addressed to U.N. High Commissioner for Human Rights Michelle Bachelet.

The note, signed by U.S. Ambassador to the U.N. in Geneva Bathsheba Nell Crocker, warns a Russian invasion of Ukraine could bring with it abuses such as kidnappings or torture, and could target political dissidents and religious and ethnic minorities, among others.

Russia has placed more than 150,000 troops near Ukraine's borders in recent weeks, the U.S. and Western allies have estimated.

Moscow denies it plans to attack its neighbor, but is seeking a guarantee that Ukraine will never join NATO and that the Western alliance will remove forces from Eastern Europe, demands the West has refused.