(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.

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