"I don't know if they are going to give up yet but they'll give up," Trump told reporters on Tuesday.
"They are doing very poorly," he added.
A US blockade has cut Iran's financial lifeline - oil exports, and economic costs of the war have made life tougher for Iranians but President Masoud Pezeshkian told the UN General Assembly last week Tehran would never surrender to US pressure.
The rise in oil prices due to the war has also become a major issue for Trump's Republican Party heading into the November US midterm elections.
The war has led to the blockade of the Strait of Hormuz, which carried a fifth of global supplies of crude oil and LNG before the conflict.
Iran's currency has fallen to a new low, with traders in Tehran exchanging more than 2.5 million rials to the US dollar in a stark reflection of how the war has steadily eroded the Iranian economy.
The latest decline came just 27 days after the rial hit its previous record low of 2.2 million to the US dollar on September 2.
The currency has repeatedly reached new lows since the war began in February.
The Iranian economy has been under duress for years in the face of international sanctions but the US naval blockade on Iranian oil and new sanctions imposed since the start of the war have sent it into free fall.
Meanwhile, Iran's top diplomat said indirect negotiations with the US aimed at reopening the critical Strait of Hormuz have become "more serious".
Foreign Minister Abbas Araqchi told Iranian media late on Monday that the "current focus is solely on the Strait of Hormuz", the choke point of the war.
Araqchi made the comments shortly before departing New York - where he attended last week's gathering of world leaders at the United Nations General Assembly - and after meeting with Pakistani and Qatari mediators.
Araqchi said the Qatari intermediaries were expected to soon engage with the US side.
"They are set to raise the matter with the American side once more, after which the final US response will be conveyed to us," Araqchi said.
"If there is a response, the Qataris will convey it to us and a decision will be made in Tehran on that basis."
Officials, including from the US, have confirmed to the Associated Press that mediators are working with Iran and the US on trying to broker a deal to end the fighting and open the strait.
The US administration said on Tuesday that 10 individuals and entities based in Iran, Hong Kong and Pakistan face sanctions over accusations of procuring weapons and weapon components for Iran's defence ministry as it fights its now seven-month war against the US.
with Reuters
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