In private, though, the president has been more adversarial, declaring in May that all Ukrainian politicians are “corrupt,” and that the Ukrainian government “tried to take me down,” according to people familiar with an Oval Office briefing delivered by a United States government delegation that had recently returned from attending Mr. Zelensky’s inauguration in Kiev. The delegation had been impressed by Mr. Zelensky, and had encouraged Mr. Trump to assist the new government.

The White House did not respond to questions about the meeting.

Vice President Mike Pence denied last week that the delay in releasing military assistance was in any way related to Mr. Giuliani’s efforts to convince the Ukrainians to investigate matters related to the family of Mr. Biden, who is the early front-runner for the Democratic presidential nomination.

But, after a meeting with Mr. Zelensky on the sidelines of a commemoration of the outbreak of World War II, Mr. Pence said he and Mr. Trump “have great concerns about issues of corruption.” Without detailing those concerns, Mr. Pence linked them to the military assistance, telling reporters “to invest additional taxpayer in Ukraine, the president wants to be assured that those resources are truly making their way to the kind of investments that will contribute to security and stability in Ukraine.”

Mr. Zelensky’s office did not immediately respond to questions emailed after business hours in Kiev.

But Senator Christopher S. Murphy, a Connecticut Democrat who serves on the Foreign Relations Committee and met with Mr. Zelensky last week, said the Ukrainian president, a political neophyte who had been a comedian before winning the presidency, seemed confused about why the assistance was being withheld.

“The first question President Zelensky asked us was whether we knew anything about the security aid,” Mr. Murphy said. “There seems to be very little understanding from anyone in the government as to why the aid has been withheld or what they need to do in order to get it released.”