US President Donald Trump has said the United States still has one or two targets to "take care of" in Iran, including a suspected underground nuclear complex, signalling that Washington's military objectives in the country may not yet be complete.

The remarks came after Iran's supreme leader issued a fresh warning to the United States and Israel, ordering the country's armed forces to prepare an immediate and impactful response to any enemy threat.

Speaking to reporters at his Florida residence on Saturday, Trump said the US had significantly weakened Iran's military capabilities and nuclear programme but still had a few targets to address.

"We've knocked out their nuclear system. We still have to take care of, I would say, one or two sites, but we can do that in 10 minutes," Trump said.

He specifically referred to Pickaxe Mountain, a suspected underground nuclear complex near Iran's Natanz nuclear facility, saying the US would have to "take care of that".

Khamenei orders forces to update target lists

In a written message issued nearly a day earlier, Iran's supreme leader said Iran would determine the timing and terms for ending any new conflict if its adversaries resumed military or terrorist attacks.

He called for an immediate, impactful and "geographically unrestricted response" to any enemy threat, and directed the armed forces to update their target lists to bring all US and Israeli assets, both tangible and intangible, within striking range.

He further instructed the armed forces to increase the production of ammunition and weapons required for reconnaissance, support and combat operations, and called for strengthening military intelligence capabilities and developing indigenous defence capabilities.

Strikes may resume after midterm elections

The exchange of warnings comes amid growing uncertainty over whether Washington could launch another round of military strikes against Iran.

Reports have suggested the Trump administration is planning to resume strikes on Iran a day after the November 3 US midterm elections, potentially opening another phase of military confrontation between the two countries.

Trump's latest remarks have added to speculation about a possible resumption of military action, even as he has maintained that Washington is pursuing diplomatic efforts to end the conflict.

Earlier on Saturday, US Secretary of State Marco Rubio said Trump had not taken a decision on launching fresh strikes against Iran and that Washington remained open to a diplomatic solution.

Trump says talks progressing but questions Tehran's reliability

Trump also said negotiations with Iran were progressing, although he questioned Tehran's reliability.

"We're going fine, but you know they're not very reliable," he said, reiterating that Washington was in no hurry to launch another offensive because it had already inflicted significant damage on Iran's military and nuclear programme.

The remarks leave the immediate future uncertain: diplomacy is still described as open, while the threat of further strikes remains on the table.