Atlanta, Georgia - President Donald Trump has not made a decision about fresh strikes on Iran amid stalled peace talks, Secretary of State Marco Rubio said Saturday, saying the US is "open" to diplomatic solutions.

"We can achieve military objectives if we seek to," Rubio told a press conference in Atlanta, Georgia, where he was hosting G20 foreign ministers for a two-day meeting.

But Rubio added that Trump "hasn't made a decision" on further military action, amid reports that the US leader could attack after key midterm elections on Tuesday.

"I have no announcement in that regard with you today, and nor do we take any of it lightly," Rubio said.

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While he slammed Tehran for "actively and openly" targeting commercial vessels for attack, Rubio also expressed Trump's preference for a diplomatic solution to end war with Iran.

But he did not commit to a timeline for talks.

"I'm not going to characterize neither the progress on those talks nor any timeframe on it, other than to tell you that if there is a diplomatic solution, we would be open to it," Rubio said.

Iran's supreme leader on Saturday warned the US and Israel against launching new attacks, following threats to resume strikes after crucial elections.

Trump earlier warned of the potential for fresh military action after US midterms, saying previously that he was weighing up whether to "finish them off."

But any further attack would spark concern among US allies.

On Thursday, EU foreign policy chief Kaja Kallas told AFP: "We are calling for a diplomatic solution, and really, escalation does not benefit anyone."