US officials pledged Tuesday that any deal struck with Iran would include a strict monitoring regime to thwart any bid by Iranian leaders to covertly try to develop nuclear weapons.

But top US negotiator Wendy Sherman would not be drawn by lawmakers on whether Washington and its partners would seek to extend the complex talks beyond a new November 24 deadline.

"Transparency and monitoring is absolutely critical and core to any agreement. As I said, one of the pathways of greatest concern is, of course, covert action," Sherman told the Senate Foreign Relations committee.

For every measure laid out in any deal with Iran to rein in its suspect nuclear program "we will decide whether in fact an additional element of transparency and monitoring is needed over the entire duration of this agreement," she insisted.

Marathon talks between Iran and world powers in Vienna ended on July 19 after negotiators gave themselves four more months to try and bridge major gaps and strike a historic nuclear deal.

But, pressed on whether the US and its allies in the so-called P5+1 group would refuse any further extension of the talks, Sherman demurred.

"I have learned in negotiations that it is very difficult to say what will happen at the end of any given period of time," the State Department's under secretary said.

"Our intent is absolutely to end this on November 24th in one direction or another."

The skilled diplomat, who has led the US negotiating team for years, revealed the talks had only been extended for four months and not the full six allowed under an interim agreement "because we thought we would just get to month five before anything would happen."

It remains unclear when the next phase of the talks will start, or where the negotiations will be held, although talks are expected on the sidelines of the UN General Assembly in September.

"I cannot tell you today that our diplomacy will succeed, because I am not sure that it will. I can tell you that in the past six months, we have made significant and steady progress," Sherman said.

"Our goal is to structure an agreement that would make any attempt to break out of such an agreement so visible and so time-consuming that Iran would either be deterred from trying or stopped before it could succeed."

If all efforts fail, "we will have very serious decisions to make," Sherman said.

"If Iran will not reach a comprehensive agreement that cuts off all of their pathways to a nuclear weapon and that gives the international community assurance we're looking for, then we will step up right with you to additional sanctions and to considering all of the options which the president of the United States says remain on the table."