AFP answers key questions surrounding the unprecedented legal measure, which America's European allies oppose and which threatens the Iranian nuclear deal.

- What's the background to this? -

In 2015, after long negotiations, the permanent members of the Security Council (the US, China, Russia, France and Britain) plus Germany signed a historic nuclear accord with Iran.

The agreement saw the UN lift international sanctions against Iran in exchange for Tehran agreeing not to develop nuclear weapons.

The deal, signed in Vienna and known officially as the Joint Comprehensive Plan of Action (JCPOA), was later unanimously ratified by the Security Council under resolution 2231.

Under that resolution, a conventional arms embargo on Iran is due to expire on October 18. Last week, the US failed to extend that embargo indefinitely, leading it to pursue the "snapback" route.

- What's the 'snapback' clause? -

Then-president Barack Obama's administration boasted of having obtained a clause in the agreement that would allow it to reimpose, or "snapback," all sanctions if Iran failed to comply with the agreement.

The clause permitted a "snapback" without members being allowed to veto. American officials worried at the time that China or Russia may veto any attempt to reimpose sanctions.

According to resolution 2231, any state "participating" in the agreement can lodge a complaint with the Security Council if they deem another participant has significantly failed to respect the accord.

That notification starts a 30-day window where the Council must pass a new resolution that would officially lift the sanctions agreed to in resolution 2231.

If the country -- in this case the US -- that issues the initial complaint wishes instead to reinstate the sanctions, then it can veto the new resolution, thus automatically triggering the "snapback."

- Is the US still a 'participant'? -

The US and its European partners disagree over who is legally a participant.

In May 2018, President Donald Trump pulled America out of the JCPOA, denouncing it as insufficient to prevent Iran from developing a nuclear bomb.

Britain, France and Germany insist that the US gave up its right as a participant when it withdrew and therefore cannot legally enforce the "snapback."

The US argues that it can trigger a return to sanctions because it participated in the signing of the resolution, which is still technically in force.

- What happens next? -

Now that US Secretary of State Mike Pompeo has formally informed the UN that America intends to restart international sanctions against Iran, the Security Council enters uncharted territory.

The mechanism has never been used before, so it's difficult to predict what will happen.

It's safe to say, however, that the US has never before been so isolated on the Security Council over Iran before. Last week, only the Dominican Republic backed America's call to extend the conventional arms embargo.

This month's sitting president of the Council, Indonesia, is expected to consult with members about whether they think the US has the right to activate "snapback," according to UN expert Richard Gowan.

The answer will be a clear "no," at which point, Gowan predicts, the president may decide to dismiss the complaint.

That could see the US adopt an even more brazen move -- tabling a draft resolution of its own to lift sanctions that it would then veto to confirm the "snapback."

"We will then enter a very strange period in which there will be two alternate universes," said Gowan, of the International Crisis Group think-tank.

"There will be a US universe in which sanctions have been fully restored on Iran. And then there will be the universe that most other council members, and I think actually most members of the UN in general, will exist in, in which nothing of the sort has happened," he told a briefing.