Kamala Harris pledged Friday she would tighten the border and fix America's broken immigration system on her first visit to the US-Mexico frontier as a presidential candidate.

The US vice president is playing catch-up on immigration, with a majority of Americans saying they trust Republican rival Donald Trump more than her on one of the most important issues for voters ahead of November's election.

In a speech in Douglas, Arizona before a friendly audience, Harris tried to straddle a tough-on-illegal-migration line with a promise to reach across the aisle to fix an immigration system she said was broken.

"The United States is a sovereign nation, and I believe we have a duty to set rules at our border and to enforce them, and I take that responsibility very seriously," she said.

"We are also a nation of immigrants... and so we must reform our immigration system to ensure that it works in an orderly way, that it is humane and that it makes our country stronger."

Harris said as president she would revive a bipartisan border bill Trump "tanked" last year, which would add 1,500 border agents and pay for 100 machines to detect smuggled fentanyl, a synthetic opioid ravaging US communities.

And she said anyone crossing the border illegally would be barred from seeking asylum in the country.

But "hard-working migrants" who come to the US legally should be given a pathway to citizenship, she said.

"I reject the false choice that suggests we must either choose between securing our border or creating a system of immigration that is safe, orderly and humane. We can and we must do both."

She said Trump, who has repeatedly demonised immigrants as murderers, rapists and mentally ill people, is "fanning the flames of fear and division."

"The American people deserve a president who cares more about border security than playing political games and their personal political future."

Harris earlier visited the border wall in Douglas, where she met with Border Patrol officials and was photographed alongside the distinctive metal slats.

It was her first visit to the frontier since Joe Biden ceded the Democratic Party nomination to her, and Republicans have hammered her for staying away.

Arizona is one of the half-dozen battleground states expected to decide the agonizingly close November 5 election, and it is where polls show Harris may have to do the most work.

Illegal crossing

Hours before Harris spoke figures from the US Department of Homeland Security showed 425,000 non-citizens convicted of crimes are living in the United States, including more than 13,000 who have convictions for homicide.

Trump leapt on the news, saying these people had been "let out of jail, and they're roaming our streets."

Trump claimed -- wrongly -- that the numbers referred to people who had entered illegally under the Biden-Harris administration.

In fact, the figures give no details on how long these people have resided in the United States, and experts say many could have been in country for decades.

"These are people who, primarily, have already been charged and convicted and served their time," Aaron Reichlin-Melnick of the American Immigration Council told AFP.

Reichlin-Melnick added that, under Trump, there were also millions of non-citizens living in the United States, including hundreds of thousands with criminal records. 

"The only reason that they cannot be deported is because of diplomatic issues with their home country and nothing to do with the US government's policy or practices," he said.

Weak spot

Recent polls have seen Harris eating into Trump's lead on migration with voters, yet it still remains a weak spot, with record illegal border crossings under her and Biden's watch.

But Harris points to numbers that have plummeted since Biden signed an executive order in June temporarily closing the border to asylum seekers -- to around 58,000 in August from a peak of 250,000 last December.

Republicans have focused on Harris's role early in the administration when Biden tasked her with looking into the causes of illegal migration from Central America. 

Trump's campaign slammed her "disgraceful" visit, branding it "a desperate attempt to fool Americans into forgetting the chaos and devastation she has unleashed over her four years as Border Czar."

Trump has doubled down on his rhetoric targeting migrants, seeing it as appealing to his base of largely white, blue-collar voters.

In remarks Thursday, Trump repeated his claim that migrants were "infecting our country," using language Biden's team has compared to that used by Nazi Germany.