WASHINGTON (BLOOMBERG) - The United States has told allies that any Russian invasion of Ukraine would potentially see it target multiple cities beyond the capital Kyiv, according to three people familiar with the matter.

Cities that could also come under attack include Kharkiv in the northeast and Odessa and Kherson in the south, said the people, all Western officials who asked not to be identified talking about such sensitive matters.

They did not provide details on the intelligence they said underlined these calculations.

The US view is shaped in part by granular insight into the types of Russian forces and capabilities currently near Ukraine, as well as how they might be prepared to act from multiple locations, the officials said.

They did not specify what a potential attack might include, or comment on what Russian President Vladimir Putin's ultimate goal might be in taking such action, but one person said US assessments suggest any large scale invasion could be backed up by air support and cyber disruptions.

An invasion from multiple locations could essentially fence Ukraine in. Such massive attacks and the huge civilian casualties they'd likely cause would galvanise Western support for sanctions on Russia, as well as shock people in Russia, many of whom have family and cultural ties to Ukraine.

Russia has positioned significant combat forces in Belarus, on the Russian-Ukrainian border, in occupied Crimea and at sea. Moscow also supports separatists in eastern Ukraine although it denies arming them.

President Joe Biden said on Friday he's now convinced Mr Putin has decided to move against Ukraine and that an invasion - including a strike on Kyiv - could come within days.

A similar assessment to Mr Biden's public remarks was shared with key European allies on Friday, the officials said.

One said there was no way of knowing with certainty what Mr Putin will ultimately decide to do, and that even if he has made a decision it can be reversed at short notice.

Mr Putin has indicated the buildup is partly intended to pressure the West on his demands for security guarantees.

Moscow continues to deny it plans to invade Ukraine and says it is already pulling troops back from areas near the border.

The US disputes that, accusing Russia of building up its forces further over the past week. Russia is now also extending drills in Belarus which were due to end Sunday.

Russian officials last week mocked Western intelligence, after similar warnings an invasion could happen as soon as Feb 15 or 16. "It is impossible to talk seriously about such issues," Foreign Ministry spokesman Maria Zakharova said by phone on Saturday in response to a request for comment on whether Russia could potentially target a number of cities in the event of an invasion.

"Bloomberg has already published several articles about the alleged Russian invasion of Ukraine or non-invasion of Ukraine," said Ms Zakharova.

"This information has never proved accurate. It was denied both by the Russian side and refuted directly by subsequent events," she said.

The German and French governments, which had adopted a more cautious view on the prospects of a Russia move on Ukraine, have stepped up their public warnings in the past 24 hours.

"Every indication is that Russia is planning a full attack on Ukraine," North Atlantic Treaty Organisation (Nato) Secretary General Jens Stoltenberg told German broadcaster ARD.

Spokesmen for the White House National Security Council declined to comment.

Attacks on some of Ukraine's trade arteries would have significant economic consequences.

As much as 70 per cent of Ukraine's exports and imports go by sea and the Odessa region's ports handle three quarters of that.

Ukraine mostly ships by sea to and from Asia, the Middle East, Africa and the Americas.