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(CNN) —  

Joe Biden pushed back on President Donald Trump’s unproven claims that Biden and his son Hunter Biden had acted inappropriately in their dealings with the Ukrainian government during Biden’s time as vice president.

“Not one single credible outlet has given any credibility to his assertion,” Biden said Friday. “Not one single one, and so I have no comment except the President should start to be president.”

Biden said no more about the issue as he and his staff loaded into a van to depart the Indian Creek Nature Center in Cedar Rapids, Iowa, after hosting a town hall on Friday afternoon.

His remarks came as a communication between Trump and a foreign leader prompted a whistleblower complaint that is now at the center of a dispute between the director of national intelligence and Congress. The whistleblower’s complaint deals at least in part with Ukraine, The New York Times and Washington Post reported Thursday night, prompting speculation that it could be tied to reports that the president’s attorney, Rudy Giuliani, had asked Ukraine to investigate Joe Biden.

Trump pressed Ukraine’s President to investigate Hunter Biden during a call earlier this summer, a person familiar with the situation said Friday.

In 2014, while Biden was vice president under President Barack Obama, Hunter Biden joined the board of a natural gas company called Burisma owned by a Ukrainian oligarch. In 2016, while Burisma was being investigated, Western governments, including the Obama administration, called for the removal of Ukraine’s top prosecutor due to concerns about corruption in his office. Joe Biden threatened to withhold $1 billion in loan guarantees from Ukraine if the prosecutor wasn’t let go.

There is no evidence of wrongdoing by either Joe or Hunter Biden.

On Thursday, Giuliani denied asking Ukraine to investigate Joe Biden but moments later said he had done just that. Giuliani’s comments came in a heated exchange on CNN’s “Cuomo Prime Time” as Chris Cuomo pressed the former New York mayor over conversations he’d had with a Ukrainian official about Biden’s possible role in that government’s dismissal of the prosecutor – who had investigated Burisma.

Earlier Friday, Trump continued to blast news of the whistleblower complaint in the Oval Office, calling it “a ridiculous story.”

“It’s a partisan whistleblower. They shouldn’t even have information. I’ve had conversations with many leaders. They’re always appropriate,” he said, adding, “It’s just another political hack job.”

Trump said he doesn’t know the identity of the person who made the complaint.

Later Friday, Biden released a statement about the whistleblower complaint, saying: “If these reports are true, then there is truly no bottom to President Trump’s willingness to abuse his power and abase our country. … At minimum, Donald Trump should immediately release the transcript of the call in question, so that the American people can judge for themselves, and direct the Office of the Director of National Intelligence to stop stonewalling and release the whistleblower notification to the Congress.”

UPDATE: This story has been updated with additional context about the lack of evidence to support the allegations against the Bidens.

CNN’s David Shortell contributed to this report.