The United States will unilaterally declare this weekend that UN sanctions against Iran are once again in force, a move that risks increasing Washington's diplomatic isolation while also stoking international tensions.  

"Virtually all UN sanctions on Iran will come back into place this weekend at 8:00 pm Eastern Time (midnight GMT) on Saturday the 19th," said Elliott Abrams, the Trump administration's special representative for Iran.

But on this point, Washington is almost alone in the world: all the other great powers -- China, Russia and also the US' own European allies -- have challenged the claim.

How did the UN arrive at this spectacular stand-off between the leading superpower and the rest of the planet? To answer that, one has to go back at least one month.

In mid-August, President Donald Trump's administration suffered a resounding defeat at the UN Security Council when it tried to extend the embargo on conventional weapons being sent to Tehran, which was due to expire in October. 

Secretary of State Mike Pompeo made an unusually vehement attack on allies France, Britain and Germany, accusing them of "siding with Iran's Ayatollahs," and on August 20 announced a controversial move known as the "snapback," which aims to re-establish all sanctions against Tehran a month later.

The sanctions were lifted in 2015 when Iran signed on to an international agreement not to seek to build nuclear weapons.

But Trump said that the landmark accord, negotiated by his predecessor Barack Obama, was insufficient and withdrew the US from the agreement in 2018, then renewed and even strengthened Washington's bilateral sanctions.

At the moment, the United States is insisting it is still a participant in the agreement that it stormed out of, but only so that it can activate the "snapback" option.

Britain, France and Germany told the UN Security Council on Friday that UN sanctions relief for Iran - agreed under a 2015 nuclear deal - would continue beyond Sept.20, when the United States asserts that all the measures should be reimposed.

   In a letter to the 15-member body, seen by Reuters, the three European parties to the nuclear deal and long-time US allies said any decision or action taken to reimpose UN sanctions "would be incapable of legal effect." The United States quit the nuclear deal in 2018.