The 15-member council has been operating virtually amid the coronavirus pandemic so states have 24 hours to cast a vote.
The 13-year-old arms embargo is due to expire in October under a 2015 nuclear deal among Iran, Russia, China, Germany, Britain, France and the United States that prevents Tehran from developing nuclear weapons in return for sanctions relief.
In a bid to gain more council support, the United States slashed its draft resolution on Tuesday to just four paragraphs that would simply extend a weapons ban on Tehran "until the Security Council decides otherwise," saying it's "essential to the maintenance of international peace and security."
But diplomats and analysts said the draft text was still likely to fail. To be adopted a resolution needs at least nine votes in favor and no vetoes by Russia, China, the United States, Britain or France.
If the United States is unsuccessful it has threatened to try and trigger a return of all UN sanctions on Iran using a provision in the nuclear deal, even though Washington quit the accord in 2018. The move could put the already fragile nuclear accord further at risk.
It was not immediately clear how Russia, China or any other Security Council members might try to stop the United States from triggering a return of all UN sanctions on Iran, known as snapback, or if - procedurally - there is any way that 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 simply wouldn't reimpose the measures on Iran themselves.Pompeo, on a visit to Austria, said Iran must also provide full and immediate cooperation with the International Atomic Energy Agency nuclear watchdog, whose head he was meeting in Vienna.Some diplomats say the attempt to extend the ban is bound to fail and put an already fragile nuclear deal between Tehran and world powers further at risk."It makes no sense to permit the world's largest state sponsor of terrorism to purchase and sell weapons systems," Pompeo told a news conference. "I mean, that's just nuts."Pompeo said he was urging the rest of the world to support the US stance."We are going to do everything that we can within our diplomatic tool set to ensure that arms embargo doesn't expire," he said."The voting will be in the next handful of hours and we are hoping that we will be successful. When we see the results we will make the decision about how to move forward."We have been unambiguous about the fact we no intention of allowing this arms embargo to expire. None whatsoever."Pompeo welcomed the agreement between Israel and the United Arab Emirates to normalize diplomatic ties."It's an important step forward for Middle East stability," Pompeo said. "It's a good moment for the world and its security."