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