"I do think you'll see another one here in the coming days," National Security Council spokesman John Kirby told reporters, referring to another tranche in the multi-billion-dollar US program to support the pro-Western country's military.

Kirby said it was too early to say whether gains by Ukraine, forcing a hasty retreat by Russian forces in the east, signal a turning point in the overall war, which began with Moscow's full-scale invasion in February.

"I think what you're seeing is certainly a shift in momentum by the Ukrainian armed forces," Kirby said, but Ukrainian President Volodymyr Zelensky should be the one to "determine and decide whether he feels militarily they've reached a turning point."

Despite the "dramatic events..., it's war and war is unpredictable."

"What we're going to do is continue to support him," Kirby said.

The official said US weaponry had been "instrumental" in Ukraine's ability to hit back at the Russians, and that "we're in lockstep with the Ukrainians."

As for Moscow's claims that it had not suffered a defeat in the latest battle, but was simply moving its troops, Kirby said: "They're calling it a repositioning but certainly they have withdrawn in the face of Ukrainian armed forces that are clearly on the offense."