WASHINGTON/MOSCOW (NYTIMES, AFP) - Over the strenuous objections of its closest allies, the Trump administration reimposed United Nations sanctions against Iran on Saturday (Sept 19), though the weight of their repercussions is unclear without the cooperation of the world's other major powers.

In an evening statement, Secretary of State Mike Pompeo announced the "return of virtually all previously terminated UN sanctions" and, in effect, declared a 2015 nuclear accord with Iran as no longer in force.

"The world will be safer as a result," Mr Pompeo said.

He also warned that the United States "is prepared to use our domestic authorities to impose consequences" for other countries that do not enforce the sanctions. He did not elaborate.

Minutes after Mr Pompeo's statement, the Iranian ambassador to the UN, Mr Majid Takht Ravanchi, said the sanctions were "null and void".

"US' illegal & false 'deadline' has come and gone," Mr Ravanchi wrote on Twitter. He warned that the United States' "swimming against int'l (international) currents will only bring it more isolation".

On Sunday, the Russian foreign ministry condemned the unilateral declaration by the US.

"The illegitimate initiatives and actions of the United States by definition cannot have international legal consequences for other countries," the foreign ministry said in a statement.

Russia, a key ally of Iran, accused Washington of putting on a "theatrical performance" and insisted that the US statements "do not correspond to reality". It accused the US of "trying to force everyone to wear virtual reality goggles" and accept its version of events, adding: "The world is not an American computer game."

Washington's defiance has dealt a "serious blow to the authority of the UN Security Council" and showed "open contempt for its decisions and for international law as a whole", the ministry said.

"This is unacceptable, and not only for us, but for other members of the Security Council too."

Just a day earlier, Britain, France and Germany said in a letter that the sanctions - which the UN had suspended after the signing of a 2015 nuclear accord - would have no legal effect.

To underscore their fundamental opposition, the letter said all three countries would work to preserve the 2015 agreement, which they jointly negotiated with the US, China and Russia, even as Washington sought to destroy its last remnants. The Trump administration withdrew from the agreement more than two years ago.

"We have worked tirelessly to preserve the nuclear agreement and remain committed to do so," said the letter, a copy of which was viewed by The New York Times.

For Mr Trump, the penalties have both a political and international calculus. He ran in 2016 declaring that the Iran deal was a "terrible" giveaway to the country's leadership, and Saturday's move will enable him to enter the last stretch of the 2020 election declaring that he had destroyed it and punished the Iranian economy by resuming sanctions that existed before the Obama administration negotiated the deal.

And should former vice-president Joe Biden win in November, the resumption of sanctions will make it more complicated to reassemble some version of the agreement. Mr Biden would have to reverse the move, making it appear he had made a concession to Iran even as it has resumed work on its nuclear programme in reaction to Mr Trump's decision to abandon the deal.

But for the president's critics, the move underlines how his administration has splintered alliances and fractured understandings with the US' superpower adversaries, Russia and China. They had been unified in reaching the 2015 agreement. Now the US has gone its own way, and Russia and China seem poised to resume conventional weapons sales to Iran next month when an arms embargo against Iran expires, over the objections of Washington.

"The irony I see here is that Trump is actually doing the UN and multilateralism a big favour, because by invoking the snapback, he is putting on display that the kind of clumsy unilateralism that he is known for doesn't work," said professor of international law Ian Johnstone from the Fletcher School at Tufts University.

He predicted the sanctions would not be enforced by other countries and would be "met with a collective shrug".

"The US will insist that the sanctions are back on, and most other countries will say, 'No, they're not'," said Prof Johnstone, who advised Mr Kofi Annan, the former UN secretary-general.

In fact, the Trump administration's insistence on reimposing the original UN sanctions, which had been formulated during the Bush and Obama administrations to force Iran to the negotiating table, means that the US alone will enforce them. That has raised doubts over whether the sanctions will bring much, if any, additional economic pain; one Security Council diplomat compared the US sabre-rattling to pulling the trigger of an unloaded gun.

Administration officials disagree. In his statement Saturday, Mr Pompeo rapped the Security Council for refusing to extend a global arms embargo against Iran that is set to expire on Oct 18. Only the Dominican Republic voted with the US, and Mr Pompeo made clear that the US demand to snap back the broader sanctions was in direct response to the arms embargo being allowed to expire, under the terms of the nuclear accord.

"Fortunately for the world, the United States took responsible action to stop this from happening," Mr Pompeo said.

In Iran, President Hassan Rouhani declared on Saturday and Sunday to be "days of victory" in a thumb to what he described as the toothless US effort to punish his country.

"This is a historic victory for Iran," he said this week, adding that "the US is isolated and embarrassed".

The practical effect of the sanctions - beyond the breach of comity of nations - may well be overblown.

The European Union has its own arms embargo against Iran that is not set to expire until 2023, once the International Atomic Energy Agency (IAEA) rules that the government in Iran has followed the terms of the nuclear deal. The European arms embargo could be lifted sooner, but only if the IAEA formally certifies that all of Iran's nuclear activity is peaceful.

That is unlikely, after Israel conducted a raid on an Iranian warehouse two years ago that contained weapons plans dating back two decades. And in reaction to Mr Trump's withdrawal from the deal, Iran has already exceeded the limits of nuclear fuel production as defined by the accord with world powers.

Additionally, the IAEA this year accused Iran of preventing inspectors from visiting suspect sites, before Iran gave international inspectors access to those blocked sites.

Until the European arms embargo is lifted, foreign diplomats predicted that companies and business employees in Europe would continue to adhere to it - making the UN sanctions largely irrelevant for Western allies.

Instead, they may serve as a warning to China, Russia and other US adversaries that have signalled interest in selling arms to Iran.

Mr Elliott Abrams, the State Department's special envoy for Iran policy, said the UN sanctions should compel European nations to "cooperate closely with us as, and when, they see any effort by Russia, China, or anybody else to sell arms to Iran".