The UN Security Council has rejected a US attempt to extend the 13-year arms embargo on Iran, US Secretary of State Mike Pompeo acknowledged on Friday.

Iran Press/America: The embargo will now expire in October under Iran's nuclear deal 2015 known as the Joint Comprehensive Plan of Action (JCPOA).

Pompeo criticized what he called the Security Council's "failure to act decisively" as "inexcusable".

The vote, he claimed, paved the way for Iran to buy and sell conventional weapons without specific UN restrictions in place for the first time in over a decade.

"The United States will never abandon our friends in the region who expected more from the Security Council," Pompeo said, adding that the US would continue to work to ensure that Iran does not have the freedom to purchase and sell weapons that, according to him, threaten the heart of Europe, the Middle East and beyond.

The US on Tuesday had cut its draft resolution to only four paragraphs to gain more support for the embargo extension. It said it was "essential to the maintenance of international peace and security".

If the US is unsuccessful it has threatened to try to bring back all UN sanctions on Iran using a provision in the nuclear deal, even though Washington withdrew from the accord in 2018.

The move could put the already fragile nuclear accord at further risk.

It was not immediately clear how Russia, China or any other Security Council members might try to stop the US from bringing on a return of the UN sanctions, or if there is any way they can.

Diplomats warn the process would be tough and messy.

They say several countries would argue that Washington legally could not activate a snapback of sanctions and therefore would not reimpose the measures on Iran themselves.