WASHINGTON (REUTERS) - On a day when Democratic presidential candidate Joe Biden sought to fortify his foreign policy credentials in his quest to take on Donald Trump next year, he mixed it up with an Iowa voter over his son's alleged role in the Ukraine scandal that has been dogging the president.

"You're a damn liar, man," an agitated Biden told a man at a campaign event in New Hampton, Iowa, who suggested that Biden had helped his son Hunter land a lucrative position with the Ukrainian oil company Burisma.

"No one has said my son has done anything wrong," Biden told the man, who did not identify himself.

"Get your words straight, jack."

Trump's push to suggest that Biden, as vice-president, worked to aid Hunter Biden's business activities in Ukraine, is at the centre of the impeachment inquiry being conducted by the US House of Representatives.

Trump and his allies have offered no evidence to support that claim.

The heckler at the Iowa event also questioned Biden's fitness for office.

The 77-year-old former vice-president responded by challenging the man to a push-up contest or an IQ test and at one point appeared to call him "fat."

Trump's re-election campaign quickly posted the video of the exchange, suggesting that Biden had gone "berserk."

Biden was in New Hampton as part of his eight-day bus tour across Iowa as he seeks to secure his place as one of the leading contenders for the Democratic presidential nomination.

The exchange was part of what had already been an eventful day for the candidate.

His campaign drew widespread praise on social media for a Twitter video post late on Wednesday in which Biden called Trump a laughing stock among world leaders.

By mid-day on Thursday (Dec 5) it had been viewed on Twitter eight million times.

The minute-long video plays off a clip taken at a Nato summit in Britain this week that appeared to show Canadian Prime Minister Justin Trudeau joking about Trump's press appearances during a chat with other world leaders.

"The world is laughing," reads the text over that clip and others of Trump's trips abroad. "We need a leader the world respects."

Trump campaign spokesman Tim Murtaugh responded: "As the President has said, Joe Biden claims that foreign leaders have told him they want him to win the election. Of course they do, they want to keep ripping off the United States like they did before Trump became president."

Biden's campaign was also bolstered on Thursday by the endorsement of John Kerry, the former US senator, secretary of State and Democratic presidential nominee.

"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, crediting Biden's help on fighting Islamic State and striking a deal on Iran's nuclear programme.

Biden argues that his foreign policy experience as a longtime senator and then Obama's vice-president have prepared him for the White House.

But he is facing a tough primary against 14 other Democrats, including a current US senator from Massachusetts, Elizabeth Warren.

Kerry will join Biden on the campaign trail on Friday in Iowa and on Sunday in New Hampshire, a state that borders Massachusetts and where Kerry's endorsement may be particularly important.

Iowa and New Hampshire will be the first states to select a Democratic nominee next February, and Kerry won both states in the 2004 primaries.