The US believes Russian forces have not yet gained control of the Ukrainian city of Kherson, but might use it as part of a plot to move to Mykolaiv and then to Odessa, according to a senior US defense official on Thursday.

Tanks have reached Kherson, the provincial capital of roughly 250,000 people, and Russian soldiers have taken over the regional administrative building, regional governor Hennadiy Laguta announced in an online statement on Thursday.

“There’s ongoing fighting there, so we’re not ready to characterize it one way or the other,” a US official told reporters, adding that the situation on the ground was rapidly shifting.

If Kherson were to be captured, it would be the first significant urban center to fall into the hands of Russian troops.

“What they may want to do is move on (Mykolaiv) so that they can then position themselves to the northeast of Odessa in case, in fact, they want to move on Odessa, not just from the sea, but from the ground,” said the official, speaking on the condition of anonymity.

The invasion, which started eight days ago, has not seen Russian aircraft control Ukraine’s airspace. Russian troops are still stalled about 25 km (16 miles) outside Kyiv’s city center.

The official said that Russia had moved about 90% of its pre-staged combat power into Ukraine so far and about 480 missiles had been fired by Russian forces at Ukrainian targets.

U.S. intelligence satellite imagery had been impacted by poor weather over Ukraine, the official said. On Monday, publicly available satellite images showed a Russian military convoy north of Kyiv stretching for about 64 km (40 miles).

A majority of Russian missile launches had come from inside of Ukraine as Russia moved in mobile systems, the official said, adding that the United States could still not confirm the presence or use of thermobaric weapons in Ukraine.

British Defense Secretary Ben Wallace earlier on Thursday said Putin had deployed thermobaric weapons systems in Ukraine.

The U.S. official said: “We know that they have the launching systems available to them in Ukraine that could be used for rockets that have a thermobaric warhead on them. But we cannot confirm that those weapons are in Ukraine and we cannot confirm any examples of use.”