With President Vladimir Putin intensifying operations, the US Defense Department also warned that Russian strikes on civilians were mounting and that Moscow was seeking to recruit foreign fighters, notably Syrians, for the war.

But the deadly invasion has slowed to a slog, and apart from some wins in southern Ukraine, Russian forces "really haven't made any noteworthy progress in the last few days," Pentagon spokesman John Kirby said.

The United States has already deployed 12,000 additional soldiers to Europe since February, but President Joe Biden has stressed that US troops will not engage in a conflict with Russian forces in Ukraine.

Over the weekend, Secretary of Defense Lloyd Austin ordered roughly 500 additional military personnel to locations in Europe, particularly NATO's eastern flank, "to augment US forces that are already in theater," Kirby told reporters.

"These additional personnel are being positioned to respond obviously to the current security environment caused by Russia's unprovoked invasion of Ukraine, and certainly to help reinforce and bolster deterrence and defense capabilities of the NATO alliance."

Kirby said the Defense Department has assessed that of the Russian forces built up along the border -- estimated by Western nations at over 150,000 troops -- Putin has "got nearly all the mass combat power that he had assembled inside Ukraine."

With Ukrainians trying to hold the assault at bay, Russia has engaged in more long-range attacks -- a mix of bombardments, rocket launches, artillery strikes and more than 625 missiles -- to make up for their lack of movement on the ground, the Pentagon said.

"We do believe... that they are having morale problems, they are having supply problems, they are having fuel problems, they're having food problems," he added.

"They're meeting a very stiff and determined Ukrainian resistance."

Bombardments have increased around the capital Kyiv, Kharkiv, Chernihiv in the north, and Mykolaiv and Mariupol in the south, and they "are having an increased effect on civilian casualties" and destroying homes, churches, hospitals and schools, Kirby said.

"The bottom line is, more civilians are being killed and wounded," he added.

"And Mr. Putin still has a choice here: Not to escalate... but to find a diplomatic path forward and end the invasion."

With enormous firepower at Putin's disposal, the Pentagon said it was noteworthy but nevertheless unclear why he would find it necessary to bring in foreign fighters.

"We know that they're trying to recruit Syrians for the fight," a senior US defense official said.

Meanwhile concern has grown that Odessa, the country's main port and a crucial economic hub, is in Russia's sights.

Ukrainian President Volodymyr Zelensky warned Sunday that Moscow was "preparing to bomb Odessa."

But the US official said the Pentagon has "not seen any evidence of a movement on Odessa" yet.