WASHINGTON — Democratic presidential candidate Joe Biden snapped at a questioner during a campaign event in Iowa on Thursday after the man asked about his son's business dealings in Ukraine.

"You're a damn liar, man," the former vice president said in a rare flash of anger at the town hall in New Hampton, Iowa.

After first saying that President Donald Trump is "messing around in Ukraine," the man told Biden "you on the other hand sent your son over there, to get him a job and work for a gas company.

"He had no experience," the man said. "You're selling access to the president."

"That's not true and no one has ever said that," Biden responded after calling the man a liar.

Biden's son Hunter served on the board of Ukrainian gas company Burisma from 2014 to 2019 and was reportedly paid $50,000 a month.

Hunter Biden has not been accused of any wrongdoing but questions have been raised about the propriety of him taking the position while his father was vice president and dealing with Ukraine issues.

Trump is facing impeachment in the Democratic-controlled House of Representatives after asking Ukrainian President Volodymyr Zelensky to investigate the Bidens.

The 77-year-old Biden also responded forcefully to a suggestion by the questioner that he was "too old" to be running for president.

"I'm not sedentary," Biden said. "The reason I'm running is because I've been around a long time and I know more than most people know and I can get things done.

"You want to check my shape on it, let's do pushups together, man," he said. "Let's run. Let's do whatever you want to do. Let's take an IQ test."

At another point, he appeared to say "Look, fat, look, here's the deal" — but this was strongly disputed by Biden aide Symone Sanders, who tweeted that he had said: "Look, facts."

Biden is the current frontrunner in the race for the 2020 Democratic presidential nomination, according to an average of national polls by RealClearPolitics.com.

Massachusetts Senator Elizabeth Warren, Vermont Senator Bernie Sanders and South Bend, Indiana, mayor Pete Buttigieg are the only other candidates in the crowded field with double-digit support.

The first vote of the Democratic primary season is to be held in Iowa in February.

Meanwhile, former US secretary of state John Kerry, who lost to George W. Bush in the 2004 presidential election, endorsed Joe Biden on Thursday as the Democratic candidate for next year's White House race.

Kerry, 75, said the former vice president to Barack Obama and longtime senator from Delaware "is the president our country desperately needs right now."

"I've never before seen the world more in need of someone who on day one can begin the incredibly hard work of putting back together the world Donald Trump has smashed apart," Kerry said in a statement.

Kerry, a former senator from Massachusetts, served as secretary of state during Obama's second term in office. Kerry and Biden also served for more than two decades together in the US Senate.

Biden, 77, is the current frontrunner in the race for the 2020 Democratic presidential nomination, according to an average of national polls by RealClearPolitics.com.

"I don't endorse lightly," Kerry said. "Joe and I both got into public service to make our country fairer for people and make the world safer.

"I've watched Joe do exactly that as a senator, statesman, and vice president," he said.

"Joe will defeat Donald Trump next November," Kerry continued. "He's the candidate with the wisdom and standing to fix what Trump has broken, to restore our place in the world, and improve the lives of working people here at home."

Kerry said he would join Biden on Friday in Iowa, which holds the first vote in February of the Democratic primary season, and travel with him to New Hampshire, another early-voting state, on Sunday. — AFP