The Pentagon says it will “rush” Patriot air defence missiles and artillery ammunition to Ukraine as part of its new military aid package.
However Patriot systems for launching the missiles will not be sent, Defence Secretary Lloyd Austin said.
Ukrainian President Volodymyr Zelensky said Patriots were “urgently” needed to face a growing Russian air threat and “can and should save lives right now”.
On Saturday, Ukraine said Russia had carried out another massive air attack.
Authorities in Kharkiv said a hospital was damaged. Energy facilities in three regions were attacked, Energy Minister German Galushchenko said.
Russia attacked with cruise missiles, S-300 surface-to-air missiles and Iskander ballistic missiles, Ukraine said, adding that 21 were downed using aircraft, air defence systems and jamming.
Across the border, a Ukrainian drone attack on an oil refinery in the Russian region of Krasnodar appeared to have caused a large explosion, though local authorities denied significant damage.
Russian authorities said on Saturday they had shot down some 68 Ukrainian drones over Russian territory.
Speaking on Friday, Mr Austin told a news conference that the US was committing to its largest security assistance package to date and would “move immediately” to get the supplies to Ukraine.
The US was using $6bn (£4.8bn) for this purpose, he said. A source confirmed to the BBC that the $6bn was part of a $60bn aid package signed into law by US President Joe Biden on Wednesday, which also includes $1bn in more immediate aid.
The assistance would include air defence munitions, counter-drone systems and artillery ammunition but not Patriot missile systems.
“It’s not just Patriots that they [the Ukrainians] need, they need other types of systems and interceptors as well,” Mr Austin said. “I would caution us all in terms of making Patriot the silver bullet.”
Mr Austin added that he was confident that more of the missile systems would be made available for Kyiv soon. Conversations were ongoing with European partners, he said, to deliver additional capabilities.
The Chairman of the US Joint Chiefs of Staff, General Charles Brown, said the assistance should eliminate the Ukrainians’ need to ration shells on the frontline.