Russia's combat power in Ukraine has declined below 90 percent of its pre-invasion levels for the first time since its attack began, a senior U.S. defense official said on Tuesday, suggesting heavy losses of weaponry and growing casualties.

The United States has estimated Russia assembled more than 150,000 troops around Ukraine before the Feb. 24 invasion, along with enough aircraft, artillery, tanks and other firepower for its full-scale attack.

"For the first time they may be just a little bit below 90 percent," the U.S. defense official told reporters on condition of anonymity. The official did not provide evidence.

Nearly a month into the war, Russian troops have failed to capture a single major city and their advance has been halted on nearly all fronts by Ukrainian forces. Moscow has instead turned to bombarding cities with artillery, missiles and bombs. Russia denies targeting civilians.

Much of that bombardment has been focused on the southeastern city of Mariupol. The senior U.S. official said Russian naval forces have likely been firing into Mariupol from the Sea of Azov over the past 24 hours.

"That wasn't the case yesterday," the official said.

Russia has not officially updated its casualty figures since stating on March 2 that 498 servicemen had been killed and 1,597 wounded.

But since then its offensive has run into further heavy resistance from Ukraine's army and volunteer defense forces.

White House national security adviser Jake Sullivan estimated on Tuesday the number of Russian casualties was in the thousands but declined to offer a precise figure.

As the conflict takes its toll, the United States has warned that Russia might seek assistance from China. Still, the White House said on Tuesday it had not seen any evidence of China providing military equipment to Russia.

The U.S. official suggested that there were no indications Russia was yet pulling additional supplies into Ukraine.

"But we do continue to see indications that they are having these discussions, and that they are making those kinds of plans both in terms of resupply, and also reinforcement," the official said.

Putin's incursion into Ukraine has forced more than 3.5 million to flee, brought the unprecedented isolation of Russia's economy as Western nations imposed sanctions, and raised fears of wider conflict in the West unthought-of for decades.

Putin calls the conflict a "special military operation" to demilitarize Ukraine and replace its pro-Western leadership, and says it's going to plan.