"The Russians don't have the numbers necessary to do a strategic breakthrough... more to the point they don't have the skill and the ability to do it," US General Christopher Cavoli, NATO Supreme Allied Commander Europe, told journalists.

"I've been in very close contact with our Ukrainian colleagues and I'm confident that they will hold the line," Cavoli said after Ukraine's military briefed NATO's top brass.

Ukraine said Thursday it was trying to "stabilise" the front line in the northeast Kharkiv region, where Moscow has made its largest territorial gains in 18 months after launching a fast-moving offensive last week.

President Volodymyr Zelensky said Kyiv was sending yet more reinforcements to the area, and the Ukrainian army said it had managed to partially halt Russia's advance.

Moscow has seized 278 square kilometres (107 square miles) of Ukrainian territory between May 9 and 15, according to AFP calculations based on data from the Institute for the Study of War (ISW) -- the largest territorial gain in a single operation since mid-December 2022.

Cavoli said Ukraine's allies were speeding major deliveries to Kyiv's forces after Washington passed a major aid package following months of delay.

"They're being shipped vast amounts of ammunition, vast amounts of short range air defense systems and significant amounts of armoured vehicles right now," he said.