Iran and major powers will seek Monday to put the finishing touches to a historic nuclear deal, with both sides saying an accord ending a 13-year standoff lies tantalisingly within reach.

The talks, the latest set of which have dragged on for more than two weeks, aim to nail down an agreement curbing Iran’s nuclear activities to make it extremely difficult for Tehran — which denies any such goal — to develop the atomic bomb.

There had been optimism that a deal would be clinched over the weekend, but finalising a framework accord struck in April has proved difficult, with talks stumbling on the exact timing of sanctions relief and Iran’s desire to have a UN conventional arms embargo lifted.

“We have come a long way. We need to reach a peak and we’re very close,” President Hassan Rouhani said in Iran Sunday, quoted by the ISNA news agency on a 16th day of talks in Vienna.

“We are so close that if you look down from below you feel as if we have got there, but when you do get there you know there are still some steps to take.”

A German diplomatic source warned the talks “could yet fail”.

“But we really are nearly there. The decisive moment has arrived. If Tehran is ready to take the final steps then things can go very quickly,” the source said late Sunday.