A satellite image shows trucks with towed artillery at the northern end of a military convoy moving south through the Ukrainian town of Velykyi Burluk, in Ukraine, April 8, 2022. Satellite image 2022 Maxar Technologies/Handout via REUTERS
A satellite image shows trucks with towed artillery at the northern end of a military convoy moving south through the Ukrainian town of Velykyi Burluk, in Ukraine, April 8, 2022. Satellite image 2022 Maxar Technologies/Handout via REUTERS Reuters / MAXAR TECHNOLOGIES

The United States believes that Russia has started reinforcing and resupplying its troops in Donbas in eastern Ukraine, a senior U.S. defense official said on Monday.

Ukrainian President Volodymyr Zelenskiy said on Monday thousands of Russian troops were massing for a new offensive in the east, and Russia said it would not halt its military operation in Ukraine for any further peace talks.

Despite the resupply efforts in Donbas, the United States did not believe this was the start of a new offensive in the region, the official said, speaking on condition of anonymity.

"We believe that this is an effort to reinforce and resupply their forces in the Donbas, they're still north of Izyum," the official told reporters, referring to a town about 180 km (112 miles) north of the city of Donetsk.

Russian vehicles being sent to the region included command and control elements and support personnel for aircraft and infantry units, the official said.

Last week, Russia completed its withdrawal from around Kyiv and had sent some troops to Belarus to be refit and resupplied.

The official said there was evidence that some of those troops in Belarus were moving east.

Russia still had the vast majority of its battalion tactical groups in Ukraine, including more than 55 groups in the south.

Russia's defense ministry said Russian sea-launched missiles had on Sunday destroyed S-300 anti-aircraft missile systems which had been supplied to Ukraine by a European country.

The official said the United States had seen an airstrike at the Dnipro airfield that destroyed some infrastructure but did not have evidence that any S-300 missile defense system had been destroyed by Russia.

At the weekend, U.S. Defense Secretary Lloyd Austin spoke with a small number of Ukrainian troops who had been in the United States for military education when Russia's invasion started.

The troops were trained on Switchblade drones while they were in the United States and were scheduled to go back to Ukraine on Sunday.

The official was not aware of any current plans to bring Ukrainian forces into the United States for training, but Washington was looking at different options on how Ukrainians could be trained to use those type of drones.