“That’s our best estimate right now,” the official told reporters, speaking on condition of anonymity.

Twelve days after starting its invasion of Ukraine, Russian forces have largely stalled north on Kyiv and still do not control the skies over Ukraine, increasingly relying on missile and artillery strikes.

The official said Russia had fired more than 625 missiles at Ukrainian targets.

The Pentagon ordered over the weekend an additional 500 troops to Europe, which would bring the total number of American forces there to about 100,000, the official said, as the United States seeks to guard against the war’s spillover into Nato nations.

The additional troops had largely been expected and are being sent from the United States to support American troops already in the region.

“It is not based on something that we saw over the course of the weekend,” the official said.

The deployment included additional refuelling aircraft that would be sent to Greece along with an ordnance and maintenance company.

The official added that while the airspace over Ukraine was still contested, Putin still had the “vast majority” of his fighter jets and helicopters that had been amassed near Ukraine available to fly.

“We have seen no indication that he has felt compelled to flow in from elsewhere in Russia additional air force capability,” the official added.