If a nuclear agreement is struck, the resolution is a step that could make it harder for the US Congress to undo a deal

Major world powers have quietly begun talks on a UN security council resolution to lift UN sanctions on Iran if a nuclear agreement is struck, a step that could make it harder for the US Congress to undo a deal, officials said.

The talks between Britain, China, France, Russia and the United States – the five permanent members of the Security Council – plus Germany and Iran, are taking place ahead of difficult negotiations that resume next week over constricting Tehran’s nuclear ability.

The US secretary of state, John Kerry, told Congress on Wednesday that an Iran nuclear deal would not be legally binding, meaning future presidents could decide not to implement it. That point was emphasized in an open letter by 47 Republican senators sent on Monday to Iran’s leaders asserting any deal could be discarded once President Barack Obama leaves office in January 2017.

But a security council resolution on a nuclear deal with Iran could be legally binding, say western diplomatic officials, complicating and possibly undercutting future attempts by Republicans in Washington to unravel an agreement.

Iran and the six powers are aiming to complete the framework of a final nuclear deal by the end of March and achieve a full agreement by 30 June to curb Iran’s most sensitive nuclear activities for at least 10 years in exchange for a gradual end of all sanctions on the Islamic Republic.

So far, those talks have focused heavily on separate US and EU sanctions on Iran’s energy and financial sectors, which Tehran desperately wants removed and are a sticking point in the talks that resume next week in Lausanne, Switzerland, between Iran and the six powers.

But western officials involved in the negotiations said they are also discussing a draft resolution for the 15- country security council to begin easing UN nuclear-related sanctions that have been in place since December 2006.

“If there’s a nuclear deal, and that’s still a big ’if’, we’ll want to move quickly on the UN sanctions issue,” an official said, requesting anonymity.