YOUNGSTOWN, OHIO: Republican Donald Trump said on Monday he would work closely with NATO allies to defeat Islamic State militants if he wins the White House, reversing an earlier threat that the United States might not meet its obligations to the Western military alliance.

In a policy speech, Trump said he would wage a multi-front "military, cyber and financial" war against Islamic State, although it was not clear how that would differ from the Obama administration's fight with the jihadist group.

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

Trump said a newly adopted approach to fighting terrorism by the organisation had led him to change his mind and he no longer considered NATO obsolete. He was apparently referring to reports the alliance is moving toward creating an intelligence post in a bid to improve information sharing.

While Trump appeared to claim credit for prodding NATO to focus more on the threat of terrorism, the 28-nation alliance has been grappling with the issue for more than a decade. NATO invoked Article 5, its collective self-defense mechanism, for the first time in its history to offer support to the United States after the September 11, 2001, attacks.

Trump called for shutting down access to the internet and social media for those aligned with Islamic State, which holds territory in Syria and Iraq. But he said he did not want to detail military strategy because it would tip off potential foes.

"We will defeat radical Islamic terrorism just as we have defeated every threat we've faced at every age and before," Trump said, blaming his Democratic rival, Hillary Clinton, the former secretary of state, and President Barack Obama for aiding the rise of Islamic State.

In a speech in the swing state of Ohio, Trump also said that in implementing his call for a temporary ban on Muslims immigrating to the country, he would institute "extreme vetting" and develop a new screening test to try to catch people who intend to do harm to the United States.

As president, he said, he would ask the US State Department and Department of Homeland Security to identify regions of the world that remain hostile to the United States and where normal screening might not be sufficient to catch those who pose a threat.

The Clinton campaign said Trump's plan to have immigrants submit to ideological tests was "a cynical ploy to escape scrutiny of his outrageous proposal to ban an entire religion from our country and no one should fall for it."

Reading from a teleprompter, Trump said Clinton did not have the judgment and character to lead the country.

"Importantly, she also lacks the mental and physical stamina to take on ISIS and all of the many adversaries we face," he said.

Trump, a wealthy New York businessman whose volatile campaign has alienated some in the Republican establishment, faced a fresh rebuke on Monday as he falls behind Clinton in opinion polls ahead of the Nov. 8 election.

The Wall Street Journal, a leading conservative voice, said in an editorial he should fix his campaign within 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.

'STOP BLAMING EVERYONE ELSE'

"If they can't get Mr. Trump to change his act by Labor Day, the GOP will have no choice but to write off the nominee as hopeless and focus on salvaging the Senate and House and other down-ballot races," the newspaper said.

Labor Day, which falls on September 5 this year, marks the end of US summer vacations and traditionally launches the final phase of the long US election season.

"As for Mr Trump, he needs to stop blaming everyone else and decide if he wants to behave like someone who wants to be president - or turn the nomination over to Mike Pence," it said, referring to the Indiana governor, who is Trump's vice presidential running mate.

Adding to Trump's woes this week was the news, first reported by The New York Times, that the name of his campaign manager, Paul Manafort, was on secret ledgers showing cash payments designated to him of more than $12 million from a Ukrainian political party with close ties to Russia.

Manafort denied any impropriety in a statement on Monday. "I have never received a single 'off-the-books cash payment' as falsely 'reported' by The New York Times, nor have I ever done work for the governments of Ukraine or Russia," he said.

Artem Sytnik, the head of Ukraine's anti-corruption bureau, confirmed in a briefing with reporters that Manafort's name appeared on a ledger and that more than $12 million had been allocated as an expenditure, referencing Manafort.

But Sytnik said the presence of Manafort's name "does not mean that he definitely received this money."

The Clinton campaign said the news was evidence of "more troubling connections between Donald Trump's team and pro-Kremlin elements in Ukraine."

Trump has spoken favorably in the past of Russian President Vladimir Putin. Last month, he invited Russian hackers to find "missing" emails from Clinton's time as secretary of state, when she used a private email server to conduct government business, although he later described that comment as sarcasm.

The current RealClearPolitics average of national opinion polls puts Clinton 6.8 points ahead of Trump, at 47.8 percent to Trump's 41 percent. Polls also show Trump trailing in states such as Pennsylvania that are likely to be pivotal in the election.