In his second big policy speech in as many weeks, Trump said on Monday 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 organisation earlier this year drew heavy criticism from allies and even some of his fellow Republicans.
But what he called the organisation'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.
NATO has been dealing with terrorism as an alliance since after the September 11, 2001 attacks on the US.
'Extreme vetting'
In a speech delivered in the vital swing state of Ohio, Trump 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 US.
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 US and where screening might not be sufficient to catch those who pose a threat.
Reading from a teleprompter to deliver the speech, he also attacked Democratic rival Hillary Clinton on her record as secretary of state and said she lacked the judgment and character to lead the country.
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 November 8 election.
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 'making US less safe'
US Vice President Joe Biden says Trump's remark that President Barack Obama founded Islamic State had increased threats to the physical safety of American troops in Iraq.
"Trump is already making our country less safe," Biden told a crowd at an appearance with Democratic presidential nominee Hillary Clinton in Scranton, Pennsylvania, on Monday.
