Illegal immigrants in the US often get better care than the nation's military veterans, Republican presidential nominee Donald Trump has said.

"We're not going to allow that to happen any longer," he told a bikers' rally in Washington DC.

Trump did not provide any evidence for his assertion.

Last year, the billionaire sparked anger by attacking the military record of Senator John McCain, a former prisoner of war.

Trump said Sen McCain was only considered a hero because he was a prisoner of war during the Vietnam War.

He then added: "I like people who weren't captured."

Since then Trump, who never served in the military, has tried to repair the damage by frequently honouring veterans at his rallies and holding fundraising events for them.

His latest comments came at the annual Rolling Thunder motorcycle rally on Sunday, which was dedicated to remembering POWs and those missing in action.

Despite previous criticism, many in the crowd cheered Trump.

"What I like about Trump is that he is one of us. He's not a politician," 52-year-old Louis Naymik was quoted as saying by the Associated Press news agency.

Trump - who has made controversial comments on a number of issues - was speaking ahead of the 7 June California primary.

He is running unopposed in California after his Republican rivals pulled out and he reached the number of delegates needed to secure the nomination. It has yet to be formalised.

The Associated Press says Congress and many states have written an assortment of laws and policies designed to restrict government services to people in the country illegally.