Ukraine invited Florida Gov. Ron DeSantis to visit after Republicans dismissed Russian aggression as a "territorial dispute."
DeSantis, widely expected to run for his 2024 presidential election, made comments in response to questions about the Republican nominee.
The former congressman said continued U.S. support for Ukraine does not count among the country's "vital national interests".
This comment indicates that he is likely to scale back his aid if he becomes president.
DeSantis also lined up with former President Donald Trump, the front runner for the Republican nomination. Donald Trump has spoken out against US support for Kyiv and criticized the Biden administration's handling of the war. Her comments point to a rift within the Republican Party between isolationists skeptical of providing military aid and mainstream party policies that support Kyiv.
In response to a poll by Fox News host Tucker Carlson, DeSantis said, "While the US has many vital national interests... becoming further entangled in a territorial dispute between Ukraine and Russia is not one of them." he said.
The 44-year-old has not officially announced his intention to challenge Trump for the Republican nomination, but he has taken all necessary steps to demonstrate his intention to do so.
Asked similarly whether US support for Kyiv is essential for Washington, Trump said, "No, it is for Europe. But not for the United States."
Ukrainian Foreign Ministry spokesman Oleg Nikolenko criticized DeSantis' remarks and tweeted an invitation to the Republican governor on Tuesday.
"We are sure that as a former military officer deployed to a combat zone, Governor Ron DeSantis knows the difference between a 'dispute' and war," Nikolenko said.
"We invite him to visit Ukraine to get a deeper understanding of Russia's full-scale invasion and the threats it poses to US interests."
While studying law at Harvard University, Mr. DeSantis was commissioned as an officer in the U.S. Navy and assigned to its legal arm, the Chief of the Attorney General (JAG) Corps.
His services as a JAG officer included working with detainees at Guantanamo Bay and serving as legal counsel to the elite U.S. Navy SEALs stationed in Iraq.
Many Republicans, including the party's Senate leadership, have long argued that defending Ukraine is in America's best interest. South Carolina Senator Lindsay Graham said DeSantis' remarks showed "a misunderstanding of the situation" on the part of the governor.
"This is not a territorial conflict, it's a war of aggression. To say it doesn't matter is to say war crimes don't matter," Graham said.
Former Vice President Mike Pence also called on the United States to step up its support.
But there are voices within the party, mainly in the House, who are hesitant to offer more support. Republican House Speaker Kevin McCarthy, who supports Ukraine, recently warned that the country may have "can't be a blank cheque" given the size of the US debt.
Politicians from both parties in parliament voted in favor of Western aid to Ukraine, with him approving more than $112bn (£92.47bn) in 2022 alone.
The United States is the largest contributor to Ukraine in terms of funds spent. It pays for drones, tanks, missiles, and other ammunition systems, as well as training, logistics, and intelligence support.
Humanitarian assistance includes food aid, clean drinking water, medical supplies, and other essential items for conflict-displaced Ukrainians. The Ukrainian government continues to provide financial aid by paying civil servants, medical staff and teachers.
