Trump also said his peace initiative with North Korea was already paying off despite criticism that his Singapore summit with North Korean leader Kim Jong-un earlier this month was long on positive imagery but short on specific accomplishments.

Trump, who was in Las Vegas to lend support to US Senator Dean Heller of Nevada, a Republican who is facing a stiff challenge for re-election, has been under fire for a policy that separates children from their parents when they illegally cross the US border with Mexico.

Amid a fierce outcry, Trump reversed himself on Wednesday and signed an executive order to abandon the policy, but the fate of more than 2300 children already separated from their parents before the order was enacted is unknown.

"My people are actually doing a very good job," Trump said in a speech at the Nevada Republican Party state convention at Suncoast Hotel and Casino in Las Vegas on Saturday.

Trump said that with a 3.8 per cent jobless rate, the US needs immigrants for jobs that need to be filled, but he wants them to be legal immigrants.

"We need people to come in, but they have to be people that love this country, can love our country and can really help us to make America great again," he said.

Trump has drawn some criticism from national security analysts for an agreement that emerged from his Singapore summit with Kim that had few details on how Pyongyang would surrender its nuclear weapons and ballistic missiles.

Trump said he believes North Korea will denuclearise and that the agreement is also paying off in terms of the North's suspension of nuclear weapons tests and missile tests, as well as the planned surrender of the remains of American soldiers killed in the 1950s Korean war.

"North Korea has tremendous potential," said Trump. "Chairman Kim sees that and a lot of tremendous things are going to happen."

Addressing the Senate race in Nevada, where Democratic US Representative Jacky Rosen is challenging Heller, Trump took aim at a favourite Democratic target, US Senator Elizabeth Warren of Massachusetts, who was campaigning with Rosen elsewhere in the state.

Trump revived a derogatory nickname for Warren, "Pocahontas", and said he had a nickname for Rosen as well: "Wacky Jacky".

Rosen fired back quickly, saying on Twitter, "Is that the best you've got, @realdonaldtrump? Let's fight back. Chip in now," with a link to contribute to her campaign.

Trump on Saturday also defended his proposed tariffs against Chinese products and his threat to imposed tariffs against European allies and said he will raise the issue once again of whether NATO allies are spending enough on defence at a NATO summit in Brussels next month.

"The trade stuff is coming along, just starting. But it's happening," Trump said. "We're a piggy bank that everybody likes to steal from."

Australian Associated Press