WASHINGTON - US Democratic presidential nominee Hillary Clinton has retained her lead over Republican rival Donald Trump despite a recent surge in rumours about her health, a new poll shows.

According to a Reuters/Ipsos poll released on Friday, Clinton held a 4-point lead over the New York businessman, 42 percent to 38 percent.

In a four-way race with Libertarian nominee Gary Johnson and Green Party candidate Jill Stein, Clinton still remained ahead while Johnson slotted at third with 7 percent and Stein was the last with 2 percent.

The result is particularly interesting as it comes days after Clinton nearly collapsed and stumbled while getting into a van after abruptly leaving a 9/11 memorial on Sunday, refuelling health rumours that have been haunting her since she suffered a blood clot in the brain four years ago.

The poll was conducted after the 68-year-old candidate announced she would take a few days off because of pneumonia.

If elected, 70-year-old Trump would be the oldest president to ever win the Oval office, while Clinton, 68, would be the second oldest.

A separate Reuters/Ipsos poll conducted earlier this week indicated that the issue of health would make “no difference” to how most American adults would vote.

The two candidates’ unpopularity still remained an issue, according to the poll, where one out of every five likely voters supported none of them for president.

Meanwhile, at the same point during the 2012 election cycle, about one out of every 10 likely voters wouldn't support then Democratic nominee Barack Obama or his Republican challenger Mitt Romney.

A tumultuous presidential election cycle has painted Clinton and Trump as two of the most unpopular candidates to have ever run for the White House.

TRUMP SUGGESTS HILLARY'S SECRET

 SERVICE AGENTS DISARM

Donald Trump raised the threat of violence against his Democratic rival Hillary Clinton again, suggesting her Secret Service guards voluntarily disarm and "see what happens to her."

Members of Clinton's government-appointed guard detail should abandon their weapons because she wants to "destroy your Second Amendment," Trump said, referring to the US Constitution's clause that enshrines the rights of Americans to bear arms.

"I think that her bodyguards should drop all weapons," the real estate billionaire told a cheering rally in Miami. "I think they should disarm. Immediately.

"What do you think. Yes? Take their guns away. She doesn't want guns. Take them. Let's see what happens to her. Take their guns away, OK. It will be very dangerous."

Secret Service agents protect both Trump and Clinton as presidential nominees.

Clinton campaign manager Robby Mook condemned Trump's remarks, saying they "should be out of bounds for a presidential candidate."

"Whether this is done to provoke protesters at a rally or casually or even as a joke, it is an unacceptable quality in anyone seeking the job of commander in chief," he said in a statement.

"He is unfit to be president and it is time Republican leaders stand up to denounce this disturbing behaviour in their nominee."

Although Clinton favours some gun control measures, Trump has inaccurately suggested she wants to ban all guns.

The Republican's statements on Friday echoed comments he made at a rally last month, when he suggested that "Second Amendment people" - gun owners or those backing gun rights - could stop Clinton from winning the White House and picking new US Supreme Court justices.

"If she gets to pick her judges, nothing you can do folks," Trump said. "Although the Second Amendment people - maybe there is, I don't know."

Trump has also previously accused Clinton of hypocrisy for receiving protection from armed Security Service agents.

"Crooked Hillary wants to get rid of all guns and yet she is surrounded by bodyguards who are fully armed," he tweeted. "No more guns to protect Hillary!"

AFP adds: On Saturday, Trump took his campaign to Texas, a state still up for grabs on Election Day, where he bashed Hillary Clinton's immigration policies as too lax.

As president, the Democratic candidate would virtually end border enforcement and place the country "in grave peril," her Republican rival said.

He was speaking before a sympathetic audience in a Houston suburb that included people who said their friends or family members had been killed by undocumented immigrants. Many wore T-shirts emblazoned with the words "A Stolen Life."

The issue has been core to Trump's campaign since he said last summer that many Mexican immigrants were drug smugglers and rapists.

Using unusually subdued tones in the presence of victims' families, the real estate tycoon asserted that Clinton would "implement amnesty by executive order, violating our constitution and putting the entire nation in grave peril."

Clinton has called for a softening of immigration practices, saying she would deport only violent criminals and terrorists, a position in stark contrast to Trump's vow to build a border wall and deport huge numbers of the undocumented.

But his suggestion that immigrants are disproportionally responsible for serious crime appears to be refuted by several studies.

Trump, however, insisted Saturday that Americans are being killed on a daily basis by the undocumented.

"Every day our border remains open, innocent Americans are needlessly victimized and killed," he said.

"Every day we fail to enforce our laws... a loving parent is at risk of losing their child."

While Trump repeated his charge that Clinton would introduce "total amnesty in the first 100 days, which means Obamacare, Social Security and Medicare for illegal immigrants," her website does not mention amnesty.

It does promise to "introduce comprehensive immigration reform with a pathway to full and equal citizenship within (the) first 100 days," lifting the threat of deportation in many cases.