The US will submit a UN Security Council resolution next week to extend an arms embargo on Iran despite opposition from Russia and China, Secretary of State Mike Pompeo said on Wednesday.
The ban on conventional weapons sales to Iran ends on October 18 and the US has threatened to force a return of UN sanctions if it is not extended.
Mr Pompeo said Washington would submit the long-awaited resolution next week.
He said he was alarmed at indications that China was already preparing arms sales to Iran.
"There are nations lining up to sell weapons that will destabilise the Middle East, put Israel at risk, put Europe at risk, risk American lives as well," Mr Pompeo said.
"We're not going to let it happen. And so we're using every diplomatic tool we have in the toolkit."
Russia and China hold power of veto in the Security Council and want the embargo to expire as laid out under a 2015 resolution.
That resolution approved a denuclearisation deal with Iran negotiated by then US president Barack Obama, from which President Donald Trump pulled out in 2018.
But Mr Pompeo says the US remains a "participant" in the accord as it was listed in the 2015 resolution, and can therefore force a return to sanctions if Iran breaches its terms.
He gave Iran's support of the Houthi rebels in Yemen as an example.
Even European allies of the US have been sceptical of whether Washington can force sanctions, and warn that the attempt may rob the Security Council of legitimacy.
The Europeans support extending the embargo but say the priority should be to preserve the nuclear accord, which is backed by Joe Biden, Mr Trump's Democratic rival in the November presidential elections.
Iran says it has the right to self-defence and that a continuation of the ban would mean an end to the nuclear deal.
Its Foreign Minister, Javad Zarif, said last month that his government was negotiating a 25-year accord with China, the main country that has defied US sanctions on Tehran.