President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE on Monday defended the idea of raising corruption issues with foreign leaders as he faces mounting scrutiny over whether he pressured the president of Ukraine to investigate his political rival.

Trump told reporters at the United Nations headquarters in New York City that it was fair to ask about corruption when determining whether to provide aid to a foreign country, even as Democrats have raised concerns that the president threatened to withhold aid to Ukraine if it didn't investigate former vice president Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE and his son.

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"We’re supporting a country. We want to make sure that country’s honest," Trump said when asked about his call with Ukrainian President Volodymyr Zelensky. "It’s very important to talk about corruption. If you don’t talk about corruption — why would you give money to a country you think is corrupt?"

"One of the reasons the new president got elected is he was going to stop corruption," Trump said of Zelensky. "So it’s very important that, on occasion, you speak to somebody about corruption.”

The president has been under increasing criticism about the July 25 call with Zelensky since reports first emerged last week that an intelligence community whistleblower filed a complaint involving Trump's interactions with the leader of Ukraine.

Multiple outlets later reported that Trump pressured Zelensky on the call to investigate Biden's son. The former vice president is leading Trump in multiple polls in a hypothetical 2020 match-up.

Rather than deny that he raised the topic with Ukraine, Trump has attacked his critics and defended his conduct on the call, calling it "perfect" and appropriate and saying he is considering releasing a transcript of the conversation.

“The conversation I had was largely congratulatory, was largely corruption, all of the corruption taking place," Trump said Sunday. "It was largely the fact that we don't want our people like Vice President Biden and his son creating to the corruption already in the Ukraine."

The controversy has led to renewed talk from Democrats about whether to launch impeachment proceedings.

Trump downplayed any talk of impeachment at the U.N., telling reporters he's "not at all seriously" concerned about the prospect.

"We had a perfect phone call with the president of Ukraine," he said. "It’s just a Democrat witch hunt, here we go again."

"The one who’s got the problem is Biden," he added.

A short time after speaking to reporters, Trump tweeted critically of the whistleblower, questioning if the individual is "on our Country's side."

"Where does he come from," tweeted Trump, who has said he does not know the whistleblower's identity. "Is this all about Schiff & the Democrats again after years of being wrong?"

The president was referencing House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.).

—Updated on Sept. 24 at 7:43 a.m.