"We don't have a plan B. We are confident in plan A," Kuleba told CNN in an interview.
"Ukraine will always fight with the resources given to it. And...what is given to Ukraine is not charity. It's an investment in the protection of NATO and in the protection of the prosperity of the American people," he said.
Kuleba said that if Russia prevailed in the conflict, now more than 22 months old, "other leaders will be tempted to follow in Russia's footprints. And ensuring security in this part of the world and deterring these leaders will require a much, much higher price tag for the United States."
Kuleba said any officials suggesting that Russian President Vladimir Putin would never attack a NATO member state if the Kremlin succeeded in Ukraine "are making a huge mistake. And they should change their job."
The U.S. administration's request for another $61 billion in support for Ukraine has been bogged down in the U.S. Congress, where Republicans say it must be paired with tougher immigration controls along the U.S.-Mexico border.
Independent U.S. Senator Kyrsten Sinema, of Arizona, which is among the states that border Mexico, said on Wednesday that Senate negotiators were "closing in" on a bipartisan border security deal, which Congress could couple with new emergency aid for Ukraine and Israel.
Still, questions remained about the Republican-controlled House of Representatives, which has been demanding Senate approval of a sweeping immigration and border security bill it passed last year.
Ukrainian President Volodymyr Zelenskiy visited Washington last month to persuade Congress to agree to the $61 billion aid package.
(Reporting by Ron Popeski; Editing by Leslie Adler)
