Donald Trump called Monday for "extreme vetting" of immigrants seeking admission to the United States, vowing to block those who sympathize with extremist groups or don't embrace American values. He said the policy would first require a temporary halt in immigration from dangerous regions of the world.

Speaking in swing state Ohio, Trump also said his Democratic rival Hillary Clinton lacks the "mental and physical stamina" to take on the Islamic State. He said destroying the terror group would be the centrepiece of his foreign policy and he would partner with any countries that share that goal — specifically singling out Russia as a nation the U.S. could have a better relationship with.

"Any country that shares this goal will be our allies," Trump said. "We can never choose our friends, but we can never fail to recognize our enemies."

Ahead of Trump's address, Clinton and Vice-President Joe Biden vigorously challenged the Republican nominee's preparedness to be commander in chief. Biden called Trump's views "dangerous" and "un-American" and warned that Trump's false assertions last week about President Obama founding the Islamic State could be used by extremists to target American service members in Iraq.

"The threat to their life has gone up a couple clicks," he said.

Hillary Clinton's campaign slammed Trump's campaign manager for ties to Russia and pro-Kremlin interests, an apparent reference to a New York Times story published Sunday night. (Andrew Harnik/Associated Press)

In his speech in the former industrial hotbed of Youngstown, Trump said he would work closely with NATO allies to defeat Islamic State militants, reversing an earlier threat that the United States might not meet its NATO obligations.

Trump said he would wage a multi-front "military, cyber and financial" war to defeat Islamic State.

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

But what he called the organization's newly adopted approach to fighting terrorism had led him to change his mind and he no longer considered NATO "obsolete," Trump said. He was apparently referring to reports the alliance is moving toward creating a new intelligence post in a bid to improve information sharing.

Blames Obama, Clinton for ISIS

NATO has been dealing with terrorism as an alliance since after the Sept. 11, 2001 attacks on the United States.

While Trump has been harshly critical of Obama's handling of the threat posed by the Islamic State, his own policies for defeating the group remain vague. His most specific prescriptions centred on changing U.S. immigration policy to keep potential attackers from entering the country.

Trump's campaign aides said the new ideological test for admission to the United States would vet applicants for their stance on issues like religious freedom, gender equality and gay rights. The government would use questionnaires, social media, interviews with friends and family or other means to determine if applicants support American values like tolerance and pluralism. The U.S. would stop issuing visas in any case where it cannot perform adequate screenings.

Trump did not clarify how U.S. officials would assess the veracity of responses to the questionnaires or how much manpower it would require to complete such arduous vetting. Nor did the campaign say whether additional screenings would apply to the millions of tourists who spend billions of dollars visiting the United States each year.

The Republican nominee's foreign policy address comes during a rocky stretch for his campaign. He's struggled to stay on message and has consistently overshadowed his policy rollouts, including an economic speech last week, with provocative statements, including his comments falsely declaring that Obama was the "founder" of the Islamic State.

The Wall Street Journal, a leading conservative voice, said in an editorial he should fix his campaign in the next three weeks or step down. Echoing growing alarm about Trump's candidacy among many leading Republicans, the newspaper said Trump had failed to establish a competent campaign operation.

Trump spent much of Monday's speech building a case that Obama and Clinton are to blame for the creation of the terror group that has roiled the Middle East and carried out attacks in the West. He specifically highlighted the withdrawal of U.S. troops from Iraq in late 2011, arguing the move created a vacuum for terror groups to thrive.

Trump's immigration proposals were the latest version of a policy that began with his unprecedented call to temporarily bar foreign Muslims from entering the country — a religious test that was criticized across party lines as un-American. Following a massacre at a gay nightclub in Orlando, Fla., in June, Trump introduced a new standard, vowing to "suspend immigration from areas of the world where there is a proven history of terrorism against the United States, Europe or our allies, until we fully understand how to end these threats."

That proposal raised numerous questions that the campaign never clarified, including whether it would apply to citizens of countries like France, Israel, or Ireland, which have suffered recent and past attacks. Trump had promised to release his list of "terror countries" soon.