Former US president Donald Trump said at a campaign event that he would scale back aid to Ukraine if he is elected.

"I will have that settled prior to taking the White House as president-elect," said Trump, the presumptive Republican nominee in the November election, while speaking at a rally in Detroit, Michigan, on Saturday.

Trump also criticized Ukrainian President Volodymyr Zelensky, calling him "the greatest salesman of all time".

"He just left four days ago with $60 billion, and he gets home, and he announces that he needs another $60 billion. It never ends," said Trump, who has said that funding for Ukraine should be more in the form of loans, and that European nations should contribute more.

At the G7 summit in Fasano, Italy, on Thursday, US President Joe Biden and Zelensky signed a 10-year bilateral security agreement.

The 10-year US-Ukraine agreement, which is not considered a treaty and did not require congressional approval, could be discarded if Trump wins in November.

"If Trump is inaugurated at noon on the 20th of January next year, by about five after noon he could have dissolved this agreement in its entirety," said John Bolton, a former national security adviser to Trump, CNN reported.

In his first term, Trump withdrew the United States from the 2015 Paris Agreement on the climate and from a 2015 nuclear agreement with Iran.