By Jason Lange

WASHINGTON, Aug 17 (Reuters) - President Donald Trump's approval rating fell to the lowest level of his presidency with an overwhelming majority ‌of Americans concerned the U.S. war with Iran will last a long ‌time, according to a Reuters/Ipsos poll that concluded on Monday.

Just 33% of respondents in the four-day survey ​said they approved of Trump's performance in the White House, while 64% disapproved. Trump's approval rating, down from 35% in a poll that closed earlier this month and lower than at any point in his current term, has now tied the lowest ‌level of his prior term ⁠reached in December 2017.

After returning to the White House last year with just under half the country approving of his presidency, ⁠Trump's popularity this year took a hit after he ordered strikes on Iran alongside U.S. ally Israel. The ensuing conflict paralyzed a fifth of the global oil trade, triggering ​a surge ​in the price of gasoline which is ​weighing on U.S. households - and ‌on Trump's Republican allies defending congressional majorities in November midterm elections.

Trump, who campaigned on promises to keep inflation in check and avoid long-lasting wars, initially pledged the conflict with Iran would take a few weeks, and argued the war was the only way to prevent Iran from developing a nuclear weapon that could threaten the ‌world.

But Iran has proved resilient and has kept ​the oil trade through the Strait of Hormuz ​largely bottled up even as ​the conflict has cooled.

Some 80% of Americans - including 87% of Democrats ‌and 71% of Republicans - think U.S. ​involvement in Iran "will go ​on for an extended period of time," the Reuters/Ipsos poll found. Just 16% said the conflict would likely end in a few weeks.

The Reuters/Ipsos ​poll, which was conducted online, ‌gathered responses from 1,166 U.S. adults nationwide and had a margin of ​error of 3 percentage points in either direction.

(Reporting by Jason Lange; ​editing by Scott Malone and Chizu Nomiyama)