The US has made the "difficult but necessary" decision to give priority to deliveries of air-defence missiles to Ukraine, while other countries will have to wait longer for weapons sales, the White House said on Thursday.
National Security spokesman John Kirby said the move is critical for Ukraine to maintain its badly needed stockpile of Patriot and Nasams air defence systems as it continues to fend off Russia's invasion.
"The United States government has made the difficult but necessary decision to re-prioritise near-term planned deliveries of foreign military sales to other countries, particularly Patriot and Nasams missiles, to go to Ukraine instead," Mr Kirby told journalists on a call.
"Deliveries of these missiles to other countries that are currently in the queue will have to be delayed."
He said Taiwan and Israel would not be affected by the decision.
Ukraine should receive the new air defences in the coming weeks, Mr Kirby said.
He said it should fulfil Kyiv's needs through the next fiscal year.
The Pentagon said Russian attacks on Ukrainian infrastructure were behind the timing of the decision.
"What we're seeing is Russia once again trying to destroy Ukraine's energy system and infrastructure ahead of winter, and so they urgently need … additional air defence capabilities," Pentagon spokesman Maj Gen Pat Ryder told reporters.
The decision to fast-track the weapons to Ukraine "will increase their inventories more quickly to enable them to continue to defend that critical infrastructure and the civilian population as we go into the winter".
Deliveries to other countries should resume in about 16 months and all countries should receive their orders eventually, Mr Kirby said.
He said the decision comes at "a key moment" in a war that has now dragged on for more than two years.
"We're going to make sure that we give Ukraine the critical air defence capabilities they need now and into the future," Mr Kirby said.
"And we're going to keep working with our allies and partners to make sure that they, too, get the air defence capabilities that they ordered."
On Wednesday, Russia's President Vladimir Putin met Kim Jong-un in Pyongyang, where the two signed a mutual defence pledge that could see North Korea sending more arms that Russia can use in its war in Ukraine.
The US and its allies say they fear Russia could provide aid for North Korea's missile and nuclear programmes, which are banned by UN Security Council resolutions.
"I just think it demonstrates the desperation that a country like Russia needs to align itself with the DPRK to subjugate the people of Ukraine," Maj Gen Ryder said, using the official abbreviation for North Korea.
"The fact that that they have to go to a country like the DPRK to obtain munitions demonstrates how isolated Russia is right now."
The US has said that Pyongyang has in the past provided Russia with ballistic missiles and artillery shells that have been used in Ukraine.
Moscow and Pyongyang have denied sending weapons to each other.
Thomas Watkins contributed to this report.
