Donald Trump signed "hard hitting" sanctions against Iran's supreme leader Ali Khamenei in the latest round of a stand-off that has brought the United States and the Islamic Republic to the brink of war.
The US president said the sanctions will deny access to "key financial resources and support" for the supreme leader, his office and those "closely affiliated" with him.
Steven Mnuchin, the US Treasury secretary, said that while the sanctions had been prepared for some time they were enlarged as a result of recent Iranian hostile actions.
"We do not ask for conflict," Mr Trump said as he signed the order, adding: "Never can Iran have a nuclear weapon."
Iran and the United States very nearly plunged into open warfare last week after Iran shot down a US surveillance drone over the Strait of Hormuz.
On Friday Mr Trump said he had cancelled retaliatory military strikes at the last minute after learning they could kill 150 people, but promised new sanctions instead.
Tensions have been building since Mr Trump pulled the United States out of 2015 agreement that granted Iran sanctions relief in exchange for curtailing its nuclear program last year, saying it would not prevent the Islamic republic from developing a bomb.
His administration has imposed a series of punishing economic sanctions in a bid to force Tehran to negotiate a new deal.
Iran has said it will begin to breach the deal by the end of this week unless European signatories relieve the economic pressure.
The new sanctions came as Mike Pompeo, the Secretary of State, launched a diplomatic blitz to build a global "coalition" around American Iran policy.
Mr Pompeo met Saudi Arabia's king Salman in Riyadh on Monday and is expected to fly on to the United Arab Emirates, India, Japan and South Korea in coming days. John Bolton, Mr Trump's national security adviser, met his Israeli and Russian counterparts for an unusual meeting in Jerusalem on Monday.
Mr Trump on Monday told China and Japan to protect their own oil shipments through the Strait of Hormuz, warning that Asian economies would be hardest hit by conflict there.
He wrote on Twitter: "China gets 91 % of its Oil from the Straight, Japan 62%, & many other countries likewise. So why are we protecting the shipping lanes for other countries (many years) for zero compensation. All of these countries should be protecting their own ships on what has always been a dangerous journey."
China gets 91% of its Oil from the Straight, Japan 62%, & many other countries likewise. So why are we protecting the shipping lanes for other countries (many years) for zero compensation. All of these countries should be protecting their own ships on what has always been....— Donald J. Trump (@realDonaldTrump) June 24, 2019
"The U.S. request for Iran is very simple - No Nuclear Weapons and No Further Sponsoring of Terror! " he added.
The United States has blamed Iran for attacks on tankers near the Strait on May 12 and June 13. Brian Hook, the U.S. Special Representative for Iran, said Washington would raise maritime security with Asian countries at the summit of G20 leaders in Japan this weekend.
"This is a global challenge that requires a global response," he said.
Mr Hook said options could included enhancing an existing multinational naval force that fights arms and drug smuggling in the region, or launching an new security initiative.
In the meantime, he said the US "campaign of diplomatic isolation and economic pressure will intensify."
Britain, the United States, Saudi Arabia, and the United Arab Emirates in a joint statement called on Iran to "to halt any further actions which threaten regional stability, and urge diplomatic solutions to de-escalate tensions."
European allies on Monday signaled they had not received any formal invitation to a US-led coalition.
Christofer Burger, a German Foreign ministry spokesman, said on Monday Berlin had "taken note via the media" of Pompeo's comments on a coalition.
"Our top aim is and remains a de-escalation of the serious situation," he added.
Andrew Murrison, the Foreign Office Minister for the Middle East, flew to Iran for talks with Iranian counterparts on Sunday, but appeared to make little progress.
Abbas Mousavi, a spokesman for the Iranian foreign ministry, said Mr Murrison's public comments suggested that "Britain is siding with America due to its domestic problems and Brexit crisis,"
Iranian media said ministers in Tehran also flatly rejected his entreaties to free jailed British-Iranian dual national Nazanin Zaghari-Ratcliffe as "blackmail."
Abbas Araghchi, a deputy foreign minister, told Mr Murrison that Iran would not "bow to blackmail" and that “she has been found guilty of spying and must stay in prison as long as convicted,” the Tasmin news agency reported.