Republican presidential nominee Donald Trump has said he will introduce "extreme vetting" of immigrants and set up a new "commission on radical Islam" if he wins the US election in November.

In a foreign policy speech on Monday, the billionaire businessman said the goal of the new commission would be to "expose" networks within the US "that support radicalisation".

As president, Trump said, he would ask the state department and department of homeland security to identify regions of the world that remain hostile to the US, and where screening might not be sufficient to catch those who pose a threat.

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"We should only admit into our country those who share our values and respect our people," he said from Youngstown, Ohio.

"In the Cold War, we had an ideological screening test. The time is overdue to develop a new screening test for the threats we face today. I call it extreme vetting."

As part of the plan, Trump, who has previously called for a ban on Muslims entering the US, said that he would "temporarily suspend immigration" from countries that have a history of "exporting terrorism".

He said that every year, the US admits 100,000 permanent immigrants "from the Middle East", and hundreds of thousands more temporary workers and visitors from the same region.

"We will stop processing visas from those areas until such time as it is deemed safe to resume based on new circumstances or new procedures."

Fight against ISIL

Outlining his policy to fight the Islamic State of Iraq and the Levant (ISIL), Trump also reversed an earlier position, promising that he would work with NATO allies to defeat the armed group if elected.

"We will also work closely with NATO on this new mission," said Trump, whose previous remarks about the organisation earlier this summer drew heavy criticism from US allies, and even some of his fellow Republicans.

Trump, who last week called US President Barack Obama "the founder" of ISIL, also attacked his Democratic opponent Hillary Clinton for enabling the rise of the armed group, which is also known as ISIS.

Sticking largely to prepared remarks and reading from a teleprompter, something he rarely does, Trump also criticised Clinton's record as secretary of state and said she lacked the judgment and character to lead the country.

'Does he have any idea?'

Trump's comments came amid increased scrutiny of his campaign and his off-the-cuff, inflammatory statements.

Before his speech, the Clinton campaign said on Twitter: "Trump's candidacy alone is undermining our national security".

In a campaign appearance with Clinton in the state of Pennsylvania, Vice President Joseph Biden said that Trump "has no clue what it takes to lead this great country".

Biden also said that Trump's accusation that Obama and Clinton had created ISIL endangered the lives of US troops abroad.

"Ladies and gentlemen, does he have any idea of the adverse consequences these comments have on our allies, our friends and the physical safety of our troops?" " he said.

"Trump is already making our country less safe."

A 'ruinous' presidency

Separately on Monday, US Asia experts who served in past Republican administrations said they would back Clinton in the presidential race, as Trump would lead to "ruinous marginalization" for the US in the region.

In an open letter, the eight former senior officials said that with global strategic competition growing, including from China, it was "absolutely the wrong time to elect an unstable, ill-prepared amateur with no vision or foresight to meet the manifold challenges of the 21st century".

They said the Republican nominee offered "only bluster or preposterous panaceas" for Asia that would "wreck our country's credibility, economy, and leadership in very short order".

The signatories to the letter included Michael Green, who served as President George W Bush's top Asia adviser at the White House, James Clad, a former deputy assistant secretary of defense, and Patrick Cronin, a former senior official at the US Agency for International Development.

"In short, if the Trump brand ... becomes America's brand, we can expect ruinous marginalization in Asia and unwanted compliance with rules which the Chinese and other challengers set," they said.