President Joe Biden on Thursday announced $800 million in new weapons for Ukraine and said the United States will support Kyiv "as long as it takes" in its war against Russian invasion.

"We intend to announce more than $800 million more" for air defence, artillery, counter battery systems and other weaponry, he told reporters at the NATO summit in Madrid.

Biden said Western support would be maintained as long as necessary and that Russia would not achieve victory.

"We are going to stick with Ukraine, and all of the alliance are going to stick with Ukraine, as long as it takes to make sure they are not defeated by Russia," he said.

"Ukraine has already dealt a severe blow to Russia," Biden said, adding that he did not "know how it's going to end, but it will not end with a Russian defeat of Ukraine."