The UN Security Council started voting on Thursday on a US bid to extend an arms embargo on Iran and the result will be announced at a meeting on Friday, diplomats said.
The 15-member council has been operating online because of the coronavirus pandemic, so states have 24 hours to cast their vote.
Russia and China, which hold the power of veto, have indicated that they are opposed to the US draft resolution.
The 13-year-old arms embargo is due to expire in October under a 2015 nuclear deal signed by Iran, Russia, China, Germany, Britain, France and the US.
The deal prevented Tehran from developing nuclear weapons in return for sanctions relief.
To try to gain more council support, the US on Tuesday cut its draft resolution to only four paragraphs, which would simply extend a weapons ban on Tehran "until the Security Council decides otherwise".
It said it was "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 favour and no vetoes by Russia, China, the US, Britain or France.
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.