A ban on selling conventional weapons to Iran ends in October under a 2015 Security Council resolution that blessed the denuclearization accord negotiated by former president Barack Obama.

"We're not going to let that happen," Pompeo told a news conference.

"In the event we can't get anyone else to act, the United States is evaluating every possibility about how we might do that."

Pompeo said he would ask the UN Security Council to prolong the ban.

But China and particularly Russia, which stand to win major new arms contracts with Iran, are certain to oppose an extension. They only agreed to the five-year ban in 2015 as a compromise reached with the Obama administration.

There is one way to avoid a veto by China or Russia -- a participant in the nuclear deal can trigger a return of sanctions by declaring Iran to be in violation.

Pompeo said that the United States will seek action from Britain, France and Germany -- which remain part of the nuclear accord.

But the US allies are critical of the US approach, saying that Europe still has a ban on arms exports to Iran and that the nuclear issue is more important.

Pompeo confirmed that the United States was ready to argue that it is itself a participant because it is listed as one in the resolution from 2015, even though Trump has repeatedly said that Washington has bolted the "worst deal ever" after he took over.

"There's nothing magic about this," Pompeo said.

"It's unambiguous, and the rights that accrue to participants of the UN Security Council resolution are fully available to all those participants," he said.

"We're going to make sure that come October of this year, the Iranians aren't able to buy conventional weapons that they would be, given what president Obama and vice president Biden have delivered to the world in that terrible deal."

Joe Biden is the presumptive Democratic presidential nominee expected to face Trump in November elections, days after the scheduled expiration of the arms embargo.

UN inspectors said Iran complied with the nuclear deal and drastically reduced its program as it sought promised sanctions relief.

But Trump, who is close to Iran's rivals Saudi Arabia and Israel, said the goal should be to reduce the clerical regime's regional activities and slapped sweeping sanctions.