"There is no alternative to the United States stepping up to the plate and providing a level of resources that allow Ukraine to have the artillery, the air defence systems and the other capabilities they need," Sullivan told a news conference alongside NATO Secretary General Jens Stoltenberg.
"Time is of the essence," he said, adding: "We're going to stay focused on that and we believe we still can and will deliver it."
Stoltenberg reinforced the message, saying: "It is vital that the United States Congress agrees on continued support for Ukraine in the near future."
"I count on all allies to sustain their commitment," he said.
Their words of support for Kyiv, made in NATO's Brussels headquarters during a visit by Sullivan, come as Republicans in the US Congress block a bill to renew American funding for Ukraine.
The Republicans, who are positioning themselves for US presidential elections in November, have tied passage of legislation to immigration curbs.
President Joe Biden has blamed Donald Trump -- his predecessor and likely rival in the upcoming elections -- for encouraging the blockage.
This month will mark the two-year anniversary of Russia's invasion of Ukraine. Kyiv's fightback is backed with material from many NATO countries, especially the United States.
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