US intelligence suggests any Russian invasion of Ukraine would employ a particularly brutal strategy to "crush" the civilian population, National Security Advisor Jake Sullivan said Monday.

The White House official also warned that the prospects of a peaceful solution were slipping through the hourglass, with Russian troops massed at the Ukraine border, and with skirmishes erupting and shells being fired by Moscow-backed rebels against Ukrainian positions in the country's eastern region.

A Russian invasion of its ex-Soviet neighbor, which Washington and the West warns could come any day, would be an "extremely violent" operation that "will cost the lives of Ukrainians and Russians, civilians and military personnel alike," Sullivan told NBC News.

"But we also have intelligence to suggest that there will be an even greater form of brutality" by Russian forces, he added.

"It will be a war waged by Russia on the Ukrainian people to repress them, to crush them, to harm them."

The United States had earlier warned the United Nations of the existence of a blacklist drawn up by Moscow of Ukrainians "to be killed" or detained in the event of an invasion, according to a letter obtained Sunday by AFP. 

Washington has "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," says the message, addressed to UN High Commissioner for Human Rights Michelle Bachelet.

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

France has proposed that Russian President Vladimir Putin and his American counterpart Joe Biden hold a summit to defuse soaring tensions, but the prospects of avoiding conflict were dimming, according to Sullivan.

"The likelihood that there is a diplomatic solution -- given the troop movements of the Russians -- is diminishing hour by hour," he told ABC News.