US President Donald Trump plans to issue an executive order allowing him to impose US sanctions on anyone who breaches a conventional arms embargo against Iran, sources said on Thursday.
They said the executive order was expected to be issued in coming days and would allow Mr Trump to impose secondary sanctions on those who breach it, depriving them of access to the US market.
A UN arms embargo on Iran is due to expire in October and the US is seeking to warn foreign entities and people that if they trade in arms with Iran, they will face US sanctions.
Under a 2015 nuclear deal that Iran signed with world powers, the UN conventional arms embargo is to set to expire on October 18, shortly before the November 3 US election.
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The US, which withdrew the nuclear deal in May 2018, says it has initiated a "snap back", or reimposition, of all UN sanctions on Iran, including the arms embargo, which would take effect at 8pm on Saturday night or midnight on Sunday.
Other parties to the nuclear deal and most of the UN Security Council say they do not believe the US has the right to trigger the snapback, which is part of the deal from which it withdrew.
A European diplomat said the executive order would strengthen Washington's ambition to keep the UN embargo in place beyond October.
Under secondary sanctions, a country punishes another for trading with a third by barring access to its own market, a powerful tool for the US because of the size of its economy.
Most foreign companies do not wish to risk being excluded from the US market in order to trade with smaller countries such as Iran.
The executive order may be more symbolic than practical because so many Iranian entities and people are already subject to the secondary penalties, said Doug Jacobson, a sanctions lawyer.
"It's essentially piling on," Mr Jacobson said. "It’s designed to send a message on this particular issue that the US is unhappy that the other parties did not agree to a snapback of arms sanctions."
Speaking on Wednesday, US special representative for Venezuela and Iran, Elliott Abrams, said Washington planned to impose sanctions on those who breached the UN arms embargo.
Also on Wednesday, US Secretary of State Mike Pompeo hinted at the coming action by stressing the power of US sanctions restored since it abandoned the Iran nuclear deal two years ago to deter foreign trade with Iran.
"We'll do all the things we need to do to ensure that those sanctions are enforced," he said.
Mr Pompeo said many experts argued that US unilateral sanctions imposed after it abandoned the nuclear deal would fail.
"We've been very successful in spite of what the world said would happen," he said.
Mr Pompeo said US sanctions had drastically reduced Iran's financial resources.