The Kremlin is "desperate" to increase its momentum in a war in which its troops have grown "frustrated and flummoxed" by a resistance that has shown durability despite being outmanned and outgunned, a senior US defense official said.

President Vladimir Putin's forces have ramped up their sorties over and near Ukraine, flying more than 300 missions in the past 24 hours, with Kyiv likewise increasing their own air operations in a bid to deny Russia superiority in the skies, the official told reporters.

Few of the operations are dogfights, as Russia's military tends to fire air-to-ground missiles at Ukrainian targets from Russian or Belarusian skies.

"They're not venturing very far or for very long into Ukrainian airspace, because the Ukrainians have been defending their airspace with great dexterity," said the official who spoke on condition of anonymity.

In the northern Black Sea, Russia is exhibiting "increased naval activity" in its use of multiple warships that are contributing to the shelling around the key port city of Odessa, he said.

But he added it was not a clear sign of an imminent amphibious assault on Odessa, where civilians have been bracing for attack.

"What we're seeing is a near desperate attempt by the Russians to gain some momentum and try to turn the course of this in their favor," the defense official said, noting that on day 26 of the offensive, Russian forces remain pinned down 15 kilometers (nine miles) northwest and 30 kilometers east of Kyiv.

At the weekend Moscow claimed it had fired hypersonic missiles at targets in Ukraine, but the Pentagon on Monday would not confirm or refute that the next-generation weaponry had been used.

The senior defense official did offer that from a military point of view using a hypersonic missile to hit a stationary target made little sense.

"It could be that they're trying to send a message to the West but also to Ukraine, and trying to gain leverage at the negotiating table," he added.

Russia also has been experiencing "a not insignificant number of failures" of its precision-guided munitions, the official said.

But the military's available combat power remains at roughly 90 percent, he added.

Ukraine's combat power is at a similar level in part because of contributions from Washington and other western powers.