IRAN accused the US yesterday of shifting the goalposts even as talks over Tehran’s nuclear energy programme neared agreement.

The current round of talks between Iran and the P5+1 group of world powers in Vienna ended its second week yesterday, having missed a target for agreement of Thursday.

Western members of the group of permanent UN security council members — and first nuclear powers — the US, Britain, France, Russia and China, as well as Germany, want assurances that Iran is not developing nuclear weapons in return for lifting sanctions.

Iranian Foreign Minister Mohammad Javad Zarif said that beyond “witnessing a change of stances” from the other side, he had noted a “different stand” on some issues among the six nations.

“This situation has made the work difficult,” he said.

On Thursday night an unnamed source told Iranian national TV that the US was refusing to accept Iran’s right to peaceful nuclear energy under 1968’s enduring Non-Proliferation Treaty.

The source said the West had “toughened its stance within the last few hours and in a clear U-turn even refuses to accept Iran’s nuclear rights.”

Another source told the Fars news agency: “While the Iranian team is showing flexibility, the Americans are refusing to accept Iran’s obvious right, particularly on sanctions.”

Mr Zarif met his US counterpart John Kerry for further talks hours after making his comments.

Mr Kerry had warned that the US was ready to leave, declaring “we can’t wait forever for a decision to be made.”

Mr Zarif claimed his side was ready to stay and work for a “dignified and balanced deal.”

Thursday’s failure to reach agreement significantly doubled the US congressional review period for a deal from 30 to 60 days.

Only after that period can US President Barack Obama lift sanctions on Iran.

Earlier on Thursday, Russian Foreign Minister Sergei Lavrov said that negotiators were close to a deal that recognised Iran’s rights to nuclear energy.

Before leaving Vienna for the joint Shanghai Co-operation Organisation-Brics summit in Ufa, Russa, on Tuesday, Mr Lavrov had said there were only eight “technical” issues remaining.