President Donald Trump says Iran will be met with "great force" if it attempts anything against US interests in the Middle East, adding that Tehran has been very hostile toward Washington.
Trump told reporters as he departed the White House for an event in Pennsylvania on Monday he was willing to have talks with Iran "when they're ready" but no discussions were going on now.
His remarks come despite urgings from Tehran for the United States to treat Iran with respect, not threats of war, in response to the US leader's comments on Sunday stoking concerns of a potential US-Iran conflict.
"With Iran, we'll see what happens," Trump said on Monday. "But they've been very hostile. They've truly been the No.1 provocateur of terror.
"I think Iran would be making a very big mistake if they did anything. If they do something, it will be met with great force but we have no indication that they will," Trump said.
"We'll have no choice," he added.
Trump warned Iranian leaders not to call for talks unless they were prepared to negotiate.
But in a sign of brewing confrontation a year after Washington quit a 2015 nuclear deal with Iran and reimposed sanctions on it, Tehran announced a fourfold increase in its rate of production of low-grade uranium enrichment.
Tensions between Washington and its Sunni Muslim Gulf Arab allies on one side and Tehran and its Shi'ite Muslim proxies in the region on the other have been flaring for weeks.
On Sunday, Trump tweeted: "If Iran wants to fight, that will be the official end of Iran. Never threaten the United States again!"
Iranian Foreign Minister Mohammad Javad Zarif replied on his Twitter account "NeverThreatenAnIranian. Try respect--it works!"
Zarif actually praised Trump for earlier remarks seen as cautioning hawks in his administration who were encouraging conflict.
The president "rightly deplores 'military-industrial complex' pushing US (hash)ForeverWars," Zarif wrote on Twitter.
But he said Trump had allowed a "B-team" of aides led by National Security Adviser John Bolton to "trash diplomacy".
He accused them of "milking despotic butchers via massive arms sales", an apparent reference to Iran's main regional foe, Saudi Arabia, Washington's biggest arms buyer.
Trump has tightened economic sanctions against Iran, and his administration says it has built up the US military presence in the region.
It accuses Iran of posing threats to US troops and interests. Tehran has denied this, describing US moves as "psychological warfare" and a "political game".
Britain told Iran on Monday not to underestimate the resolve of the US, warning that if American interests were attacked then the Trump administration would retaliate.
Quoting an official at the Natanz enrichment plant, the semi-official Tasnim said Iran was accelerating the production rate at which it refines uranium to 3.67 per cent fissile purity, suitable for civilian nuclear power generation.
