Russia has started moving very small numbers of troops away from positions around Ukraine's capital Kyiv, the Pentagon said on Tuesday, adding it was more of a repositioning than a retreat or a withdrawal from the war.

"Has there been some movement by some Russian units away from Kyiv in the last day or so? Yes, we think so. Small numbers," Pentagon spokesman John Kirby told a news briefing.

"But we believe that this is a repositioning, not a real withdrawal, and that we all should be prepared to watch for a major offensive against other areas of Ukraine. It does not mean that the threat to Kyiv is over."