DES MOINES, Iowa — Former Vice President Joe Biden said Saturday he hasn't spoken to his son Hunter Biden about his overseas businesses while forcefully calling again for an investigation into President Donald Trump's July phone call with Ukraine's president.

“I have never spoken to my son about his overseas business dealings,” Biden said. “Here’s what I know. Trump should be investigated.”

“You should be looking at Trump," Biden told reporters in Des Moines shortly after arriving at the Polk County Steak Fry, an annual Democratic fundraiser. “He’s doing this because he knows I’ll beat him like a drum. And he’s using an abuse of power and every element of the presidency to try to smear me ... Ask the right questions.”

The comments by the former vice president are the latest in a back-and-forth between Biden and Trump related to Trump's reported discussions with the Ukrainian president, a whistleblower complaint involving Trump, and Hunter Biden's business dealings in Ukraine.

Explaining the Trump, Biden, Ukraine story:What to know about the Trump-Congress standoff over a whistleblower complaint

Multiple news outlets reported this week that a whistleblower from the intelligence community filed a complaint with the inspector general overseeing that community concerning a phone call Trump made with a foreign leader.

Lawmakers are separately investigating whether Trump pushed for Ukraine President Volodymyr Zelensky to probe Biden's family's dealings in his country.

Hunter Biden, however, told the New Yorker in an interview for an article published in July that he discussed Burisma, the Ukrainian natural gas producer, with his father in December 2015. At the time, Hunter Biden sat on Burisma‘s board of directors.

Around the time of the conversation, Biden was preparing for a trip to the Ukraine and the Obama administration special envoy had raised the issue with the Vice President, according to the article. Hunter Biden told the New Yorker he and his father spoke about Burisma just once.

“‘Dad said, ‘I hope you know what you’re doing,” and I said, ‘I do,’ ” the younger Biden is quoted as saying in the article.

The Biden campaign declined to comment on the reported December 2015 conversation.

Before Biden's comments, Trump fired off several tweets Saturday morning, defending himself and his phone call with Zelensky as a "fine and routine conversation."

"Nothing was said that was in any way wrong, but Biden’s demand, on the other hand, was a complete and total disaster," Trump tweeted.

Biden briefly addressed supporters at a pre-steak fry rally Saturday and warned that Trump would try to murky the waters as the election cycle heats up.

“Be prepared for every lousy thing coming from him,” Biden said.

What has happened already

The president's personal attorney Rudy Giuliani has previously raised questions about whether Biden, as vice president, pushed for the ouster of former Ukrainian prosecutor general Viktor Shokin, who had investigated a private Ukrainian gas company, Burisma Group. Hunter Biden was a board member of Burisma.

But Ukraine's current prosecutor, Yuriy Lutsenko, told Bloomberg News Service in May that he had no evidence of wrongdoing by Biden or his son.

“Hunter Biden did not violate any Ukrainian laws — at least as of now, we do not see any wrongdoing,” Lutsenko told Bloomberg. “A company can pay however much it wants to its board.”

The Wall Street Journal reported Friday that Trump in a July phone call repeatedly pressured Zelensky to investigate Hunter Biden and work with his personal attorney, Giuliani, on the matter.

Previously:Joe Biden: Trump's reported effort to get Ukraine president to investigate him 'abhorrent'

Biden has previously said he wanted Shokin out as prosecutor general because he wasn’t doing enough to investigate corruption. The former vice president in March 2016 called on Ukraine’s Parliament to dismiss Shokin, or the United States would withhold $1 billion in loan guarantees, according to the New York Times. Shokin was voted out by parliament, but no evidence showing Biden intentionally tried to benefit his son has surfaced.

Still, Trump in a May interview on Fox News suggested Biden engaged in a quid pro quo with Ukraine's government.

“Look at Joe Biden … he calls them [Ukrainian government leaders] and says, 'Don’t you dare prosecute,'" Trump said in the interview. "'If you don’t fire this prosecutor’ — the prosecutor was after his son — then he said, ‘If you fire the prosecutor, you’ll be OK. And if you don’t fire the prosecutor, we’re not giving you $2 billion in loan guarantees,’ or whatever he was supposed to give. Can you imagine if I did that?"