"A very small number of Ukrainians... were already here in the United States going through some professional military education," the senior defense official said, requesting anonymity.

"We took advantage of the opportunity to pull them aside for a couple of days and provide them some training, particularly on the Switchblade (drones) that is a system that is not organic to the Ukrainian military," he said.

The group, made up of "less than a dozen" people, "will be heading back into Ukraine relatively soon" as originally planned, he added.

The Pentagon official had been asked about comments made the day before by Defense Secretary Lloyd Austin, who said the United States was training Ukrainian soldiers outside of Ukraine with weapons supplied by Washington.

"Some of them are being trained right here in the States," US Chairman of the Joint Chiefs of Staff General Mark Milley had said, without offering details.

On the ground in Ukraine, the totality of Russian forces that had been deployed in the Kyiv region appear to have left the country, the senior Pentagon official said Wednesday.

"We are assessing that they have completely withdrawn from Kyiv and from Chernigiv," he said.

Russian airstrikes have intensified in Ukraine's east, where Moscow indicated it intended to focus its efforts going forward.

But "we have not seen an appreciable tangible influx of additional personnel or resources applied on the ground... into the Donbas," the defense official said.