Volodymyr Zelensky, president of Ukraine, says working with Donald Trump will be difficult.
Zelensky however added that he is willing to work with anyone who wins the US presidential election.
Trump, a former US president, is the candidate of the Republican Party for the November poll.
Zelensky spoke in an interview with the BBC in London.
When Russia invaded Ukraine in 2022, Trump described President Vladimir Putin’s moves as “genius” and “savvy”.
Trump’s 2016 campaign and alleged Russian interference in that year’s election were the subject of an investigation which the ex-president repeatedly called a “witch hunt”.
Trump has also repeatedly faulted US aid — worth billions of dollars — to Ukraine as the east European nation engages in a prolonged war with Russia.
Delivering a speech at the Republican national convention, Trump said he would end the Russia-Ukraine war but did not specify how.
Asked if he had fears about Trump winning the election, Zelensky said: “It will be hard work, but we are hard workers, we are not afraid of hard work”.
Earlier this week, Trump announced JD Vance, Ohio senator, as his running mate.
Like Trump, the 39-year-old has in the past expressed indifference about the Russia-Ukraine war, saying “he doesn’t care what happens to Ukraine one way or the other”.
“Maybe he really doesn’t understand what goes on in Ukraine, so we have to work with the United States,” Zelensky said of Vance’s remarks.
Ukraine mostly relies on aid from its allies in Europe and other Western nations to bolster its defences against Russia.
Zelensky has remained optimistic about defeating Russia.
The Ukrainian president said he would consider stepping down when the war is over.