They will lose the ability to take such action on Oct. 18 next year when a 2015 U.N. resolution expires. The resolution enshrines Iran's deal with Britain, Germany, France, the United States, Russia and China that lifted sanctions on Tehran in exchange for restrictions on its nuclear program.

Iran is "dramatically" accelerating enrichment of uranium to up to 60% purity, close to the roughly 90% level that is weapons grade, the U.N. nuclear watchdog chief told Reuters last week.

The move comes as Iran has suffered a series of strategic setbacks, including Israel's assault on Tehran's proxy militias Hamas in Gaza and Hezbollah in Lebanon and the ouster of Iranian ally Bashar al-Assad in Syria.

Western states say there is no need to enrich uranium to such a high level under any civilian program and that no other country has done so without producing nuclear bombs. Iran denies pursuing nuclear weapons.

"Iran must deescalate its nuclear program to create the political environment conducive to meaningful progress and a negotiated solution," the U.N. ambassadors of Britain, Germany and France wrote in a Dec. 6 letter to the Security Council.

"We reiterate our determination to use all diplomatic tools to prevent Iran from acquiring a nuclear weapon, including using snap back if necessary," they said.

The communication was in response to letters earlier last week from Russia and Iran, which followed an initial note to the council by Britain, Germany and France on Nov. 27. Russia and Iran also then followed up with further letters this week.

The tit-for-tat letters came as European and Iranian diplomats met late last month to discuss whether they can work to defuse regional tensions, including over Tehran's nuclear program, before Donald Trump's return to the White House.

During his first term, Trump quit the nuclear deal in 2018.

'EMBRACE DIPLOMACY'

In a letter to the council on Monday, Iran's U.N. Ambassador Amir Saeid Iravani urged the Europeans to "abandon their ineffective and failed policy of pressure and confrontation," saying they "should embrace diplomacy and focus on rebuilding the trust essential to resolving the current impasse."

The European parties to the Iran nuclear deal have adopted a tougher stance on Iran in recent months, notably since Tehran ramped up its military support to Russia for its war in Ukraine.

Russian U.N. Ambassador Vassily Nebenzia - in a Tuesday letter - said Britain, Germany and France had no right to invoke the "snap back" of sanctions and that it was irresponsible of them to suggest the possibility of using the "snap back" mechanism.

U.N. Secretary-General Antonio Guterres submitted his biannual report to the Security Council on the implementation of the 2015 resolution on Tuesday, warning there was a "critical need for a peaceful solution to the Iranian nuclear issue" given the deteriorating situation across the Middle East.

The "snap back" of international sanctions on Iran would require Iran to suspend all nuclear enrichment-related and reprocessing activities, including research and development, and ban imports of anything that could contribute to those activities or developing nuclear arms delivery systems.

It would also reimpose a conventional arms embargo, ban Iran from developing ballistic missiles capable of delivering nuclear weapons and revive targeted sanctions on dozens of individuals and entities. Countries also would be urged to inspect shipments to and from Iran and authorized to seize any banned cargo.