President Donald Trump has signed an executive order to freeze billions of dollars in Iranian assets after the downing of a US drone last week.

Mr Trump said the new “hard-hitting” sanctions will “deny” Iranian Supreme Leader Ayatollah Ali Khamenei and other senior Iranian officials “access to key financial resources and support”. 

“The assets of Ayatollah Khomeini will not be spared from the sanctions,” Mr Trump said in an Oval Office signing ceremony attended by Vice President Pence and Treasury Secretary Steven Mnuchin on Tuesday (Australian time).

Mr Mnuchin said they would “lock up literally billions of dollars of assets”.

The economic sanctions were imposed as retaliation for the shooting down of an unmanned US military drone over the Strait of Hormu on June 20, Mr Trump told reporters.

“This (the sanctions) is something that was going to happen anyway,” he added. 

Mr Trump spoke a day after Iran’s navy chief warned the United States that Iranian forces could shoot down more surveillance drones if they violate the country’s airspace.

Iranian naval commander Mr Khanzadi said the downing of the drone last week could “always be repeated, and the enemy knows it,” the Tasnim News Agency reported.

Mr Trump said Mr Khamenei was ultimately responsible for what Mr Trump called “the hostile conduct of the regime” in the Middle East.

Iran’s Supreme Leader Ayatollah Ali Khamanei was held responsible for the downing of a US drone. Photo: Getty

The sanctions are aimed at denying Iran’s leadership access to financial resources, blocking them from using the US financial system or having access to any assets in the United States.

“Anybody who conducts significant transactions with these sanctioned individuals may be exposed to sanctions themselves,” the White House said.

Some policy analysts say that earlier sanctions issued under Mr Trump’s “maximum pressure” campaign are why Iran has felt compelled to adopt more aggressive tactics as its economy feels the crunch.

The Trump administration wants to force Tehran to open talks on its nuclear and missile programmes and its activities in the region.

Tensions between the US and Iran have grown since May when Washington ordered all countries to halt imports of Iranian oil. Iran says it will not be forced to the negotiating table.

“We call on the regime to abandon its nuclear ambitions, change its destructive behaviour, respect the rights of its people, and return in good faith to the negotiating table,” Mr Trump said in a statement issued along with the text of the executive order.

Iran’s hardline semi-official Tasnim and Fars news agencies said the sanctions imposed on Tehran were based on “fabricated excuses”.

Mr Mnuchin said Iranian Foreign Minister Mohammad Javad Zarif would be targeted with US sanctions later this week.

Sanctions were also imposed on eight senior commanders of Navy, Aerospace, and Ground Forces of the Islamic Revolutionary Guards Corps (IRGC), the US Treasury Department said.

“These commanders sit atop a bureaucracy that supervises the IRGC’s malicious regional activities, including its provocative ballistic missile program, harassment and sabotage of commercial vessels in international waters, and its destabilising presence in Syria,” the department said in a statement.

Mr Trump said the sanctions are a “strong and proportionate response to Iran’s increasingly provocative actions”.

“We will continue to increase pressure on Tehran until the regime abandons its dangerous activities and its aspirations,” he said.

Iran said on Monday that US cyber attacks on its military had failed, as Washington sought to rally support in the Middle East and Europe for a hardline stance that has brought it to the verge of conflict with its longtime foe.

Washington has blamed Tehran for attacks on tankers in the Gulf in recent weeks, which Iran denies.

-with AAP