The Pentagon has announced that it will "accelerate" the shipment of Patriot anti-aircraft missiles and artillery ammunition as part of a new military aid package to Ukraine.
The Patriot missile launch system will not be sent in the first round, however, said Lloyd Austin, the US defense secretary.
Ukrainian President Volodymyr Zelensky indicated that they "urgently" need the Patriots to "face the growing Russian air threat and save lives."
On Saturday, April 26, Ukraine announced that Russia had carried out another massive airstrike.
According to local authorities, energy facilities and a hospital were affected.
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Russia attacked with cruise missiles, S-300 surface-to-air missiles and Iskander ballistic missiles, Ukrainian officials said, adding that 21 were shot down by aircraft and air defense systems.
Kiev claims it hit two Russian oil refineries across the border, and Moscow shot down around 68 Ukrainian drones over its own territory on Saturday.
At the press conference, Austin said that America is committed to the largest package of military aid to date and that it will "move immediately to collect supplies" needed by Ukraine.
The US will set aside six billion dollars for that purpose, he added.
A source confirmed to the BBC that the six billion dollars is part of the $60 billion aid package signed by US President Joe Biden on Wednesday.
One billion dollars is earmarked for financing humanitarian measures.
Part of that package will be munitions for air defense, anti-drone systems and artillery munitions, but not Patriot missile systems.
"The Ukrainians need not only the Patriot, but also other types of systems and interceptors," Austin said.
He added that he is convinced that more missile systems will soon be available to Kyiv and that talks are underway with European partners to provide additional options.
Part of the latest US financial aid would also be used to build up Ukraine's defense industry, so that it can start producing more of its own munitions, which it desperately needs.
The American Minister of Defense also stated that Russia has already increased its domestic production of artillery ammunition and other weapons, as well as that it is supplied with supplies by Iran and North Korea.
"You understand what is at stake and what it means for Ukraine, Europe and America.
"If Putin wins in Ukraine, Europe would face a security threat like it has never seen before," Austin said, adding that "Russia will not stop in Ukraine."
The US minister's speech came a day after Ukraine warned of increased Russian attacks on its railways.
Zelensky recently said that they need at least seven more Patriot, or similar systems.
It is the most expensive and most capable air defense system that Ukraine has.
Each Patriot battery costs about a billion dollars, and each missile costs four times that.
Germany has already pledged an additional Patriot system, and its defense and foreign ministers appealed to their European counterparts earlier this month to act urgently.
Greece has stocks of Patriots and S-300s, but has said that no one can be spared.
"We have explained why we cannot do that," Greek Prime Minister Kyrios Mitsotakis told Sky television.
According to some reports, Spain will also deliver some Patriot missiles to Ukraine, but not the complete system.
In recent months, Kiev has stepped up appeals for help, as its ammunition stocks are depleted and Russia's are steadily growing.
Ukrainian officials blame delays in military aid from the US and other Western allies for the loss of life and territory.
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