US presidential contender Bernie Sanders has been forced to cancel campaign events until further notice after undergoing an unexpected heart procedure following sudden ill health, according to an adviser.

Key points: Experts say Senator Bernie Sanders health issues may affect his campaign

Experts say Senator Bernie Sanders health issues may affect his campaign The senator was scheduled to participate in the fourth Democratic debate on October 15

The senator was scheduled to participate in the fourth Democratic debate on October 15 The incident could renew questions about his age in a Democratic race featuring a generational divide

The 78-year-old was in Las Vegas, Nevada, on Tuesday (local time) for 2020 presidential campaign events when he experienced discomfort and was taken to a hospital for evaluation where it was found that he had a blocked artery.

"Following medical evaluation and testing he was found to have a blockage in one artery and two stents were successfully inserted," Senator Sanders' senior adviser Jeff Weaver said in a statement.

"Senator Sanders is conversing and in good spirits. He will be resting up over the next few days. We are cancelling his events and appearances until further notice, and we will continue to provide appropriate updates."

The senator, who is the oldest of 19 contenders for the Democratic presidential nomination, was scheduled to hold a town hall in Las Vegas and appear at a gun safety forum on Wednesday.

Questions about age

Aides did not offer any guidance on how long Senator Sanders might be off the campaign trail.

The first nominating contest is not until Feburary 3 in Iowa, but Senator Sanders is one of 12 candidates scheduled to participate in the fourth Democratic debate in Ohio on October 15.

The US senator from Vermont has been among the top contenders in the crowded field seeking the 2020 nomination to challenge Republican President Donald Trump.

The incident could renew questions about his age in a Democratic race featuring a generational divide between older candidates such as Senator Sanders and front-runner Joe Biden, 76, and younger contenders such as Pete Buttigieg, 37, the mayor of South Bend, Indiana.

In a letter made public during the 2016 campaign, the senator's doctor said he was in "overall good health" and he had no history of cardiovascular disease.

The insertion of stents to open blocked heart arteries is a relatively common procedure, with as many as one million Americans a year undergoing it, medical experts said.

It involves inserting a balloon-tipped catheter to open blockage and deploy tiny wire-mesh tubes to prop open the artery.

How long does recovery take?

Steven Nissen, a cardiologist at the Cleveland Clinic, said in general, recovery took a few days, but how quickly Senator Sanders bounced back would depend on his symptoms before getting the stent.

"People recover quickly, and assuming no complications, there is no reason why he could not get back on the campaign trail in a timely fashion," Dr Nissen said.

The health issue comes as Senator Sanders has been trying to turn a corner after a summer that saw him eclipsed as the premier liberal in the field by Massachusetts Senator Elizabeth Warren, 70.

Senator Sanders has dropped well behind Senator Warren and Senator Biden in most polls and recently reshuffled his staffing in early states to become more competitive.

"Given his recent stalls in the polls, the timing is pretty bad here," Democratic strategist Jim Manley said.

The campaign halt comes the day after Senator Sanders reported a big $US25.3 million ($37.8 million) fundraising haul for the third quarter, putting him in the early lead in the closely watched campaign money race.

On a telephone call with supporters, campaign manager Faiz Shakir said the "state of the campaign is strong" and touted the fundraising total and its first television ad campaign scheduled to launch in Iowa.

But those spots were suspended on Wednesday.

Several of his Democratic 2020 rivals, including Senator Biden were quick to wish him well.

Loading

Senator Sanders's wife, Jane O'Meara Sanders, was en route to Las Vegas on Wednesday and said her husband was "doing really well".

Tick Segerblom, a Clark County, Nevada commissioner who was at the senator's fundraiser on Tuesday, said the senator seemed fine at the time.

"He spoke well. He jumped up on the stage. There was just nothing visible," Mr Segerblom said.

A former mayor of Burlington, Vermont, Senator Sanders won a US House of Representatives seat in 1990, making him the first independent elected to the House in 40 years.

Senator Sanders mounted an insurgent campaign against Hillary Clinton for the party's nomination in 2016.

In 2006, he won a US Senate seat and in 2018 was voted in for a third six-year term.

Reuters/AP