It would take Iran just two months to produce enough fissile material for a nuclear weapon, US Secretary of State John Kerry said Tuesday, delivering a sober assessment on efforts to stymie Tehran’s nuclear program.

The statement came as officials from the US and other world powers met in Vienna to discuss drafting an agreement to curb Iranian nuclear activity in exchange for sanctions relief.

Speaking before the Senate Foreign Relations Committee, Kerry said it was no secret that Iran’s “breakout period” was two months, but that the so-called P5+1 talks were aiming to raise that period to half a year.

“I think it’s public knowledge today that we’re operating with a time period for a so-called breakout of about two months,” Kerry said.

“Six months to 12 months is — I’m not saying that’s what we’d settle for, but even that is significantly more,” he said of the efforts in Vienna.

Kerry noted that there was still a gap between having the material for a bomb and actually being able to build and deliver one, according to a Reuters account of the meeting.

“It’s just having one bomb’s worth, conceivably, of material, but without any necessary capacity to put it in anything, to deliver it, to have any mechanism to do so,” he said.

Kerry insisted that the US and other world powers have an “amazing capacity” to understand what Iran is doing through inspections.

Analysts have said efforts at tracking the development of Iran’s weaponization program have been less successful because it can be easily concealed until an actual nuclear test.

Iran claims its nuclear program is peaceful.

Kerry’s successor at the committee helm, Sen. Bob Menendez, a Democrat, pressed him about possible concessions by the West in the nuclear talks involving Iran and six world powers. One of the leading proponents of sanctions on Iran, Menendez sought assurances that the US would impose economic sanctions if Iran and Russia move forward with a reported oil-for-goods contract.

On the question of the fate of a heavy water plutonium reactor in Iran, Menendez expressed exasperation.

“Originally we were told that’s going to be dismantled. Now we are told we are going to find a different purpose for it. It continues to morph into different areas,” he said.

On Wednesday, the Iran and world powers were set for a second day of nuclear talks hoping to move to the next level and start drafting a historic and highly ambitious final deal next month.

Iran and the five UN Security Council permanent members plus Germany want to transform a temporary accord struck in November into a permanent agreement before it lapses on July 20.

Doing so is a tall order, however, requiring both sides to tackle thorny issues that will severely test their willingness and ability to give ground.

Iranian Foreign Minister Mohammad Javad Zarif said Tuesday after the start of the third round of talks he had formed a group of legal experts to help in the “complicated, difficult and slow work” of drafting a deal.

But his deputy Abbas Araqchi said differences had been reduced on a number of questions and the general atmosphere during the talks was good.

A fourth round of talks will be held in Vienna in mid-May, he added.

A spokesman for the powers’ chief negotiator, EU foreign policy chief Catherine Ashton, said this round was to “explore our respective positions on each topic”.

A senior US official involved in the talks said Friday she was “absolutely convinced” a deal could be reached and that both sides were “looking toward beginning drafting in May”.

Under the November deal, which took effect on January 20, Iran froze certain nuclear activities for six months in exchange for minor relief from sanctions hurting its economy.

But it has not permanently dismantled any of its nuclear equipment and could any moment stop the freeze, although this would invite new sanctions.

The deal may involve Iran slashing the number of centrifuges — used to enrich nuclear material — changing the design of a new reactor at Arak and giving UN inspectors more oversight.

The Associated Press contributed to this report.