Iran nuke deal negotiations continue amid mounting U.S. pressure for new sanctions

Negotiations to curb Iran's nuclear program resumed Friday in Geneva.

For a third day, Iran's Foreign Minister Mohammad Javad Zarif and EU chief diplomat Catherine Ashton were trying to hammer-out the sticking points holding up an agreement that would see Iran at least slow the pace of its uranium enrichment in exchange for a limited easing of economic sanctions.

A source close to the Iranian negotiating team told CBS News the talks were "moving in the right direction," but warned it was "important for all sides not to push this beyond the limit. We are still hoping for a win-win outcome. This is no time for brinksmanship."

But CBS News correspondent Margaret Brennan told "CBS This Morning" that pressure is mounting in Washington for the talks to show tangible progress, and the clock is ticking.

Senior U.S. lawmakers have been preparing to move ahead with plans to bring legislation to a vote that would impose further sanctions against Iran. Senate Majority Leader Harry Reid, D-Nev., said Thursday that he supported the diplomatic effort, but he would also support expanded sanctions.

The Democrat-led Senate signaled it would only give President Obama until next month before it presses ahead with new sanctions -- a move the White House warns could destroy the chances of a diplomatic deal.

Iranian negotiators want more significant financial relief in the form of eased sanctions than the U.S. and its allies are offering.

"I'm not in a position to go into the details of that," said Iran's deputy foreign minister Abbas Araqchi, "but we have some major differences still and those differences are on every side, almost."

Another key sticking point, according to the source close to Iran's negotiators, is Iran's insistence that it has the implicit right to enrich uranium at all. Zarif indicated in the days prior to this round of negotiations that Iran could accept a deal which makes no explicit reference to this as a "right," increasing optimism that a deal might be struck.

According to the source who spoke to CBS News on Friday, Iran is still not demanding their right be explicitly recognized in the agreement, as the Islamic Republic believes that right is implicit as Iran is a signatory to the Non-Proliferation Treaty.

"We all recognized the red line going in," said the source, adding that there were still "ifs and buts" on the matter of Iran continuing to enrich uranium that "need to be cleared up."

Iran agreed to the nuclear talks because its economy is in desperate shape. U.S.-led sanctions have cost the country $120 billion in oil sales and slashed the value of its currency.

The Treasury Department's David Cohen helps enforce those sanctions.

"The whole idea behind the sanctions, from the outset, was to create leverage," explained Cohen, "to move (Iran) away from where it's been, proceeding with its nuclear program."

But first, Iran and world powers will first have to agree on how much money is on the table. The U.S. froze $100 billion worth of the regime's assets. The Iranians want access to that money.

For now, Brennan said Iran is being offered less than $10 billion in sanctions relief if it signs a six-month deal to slow its nuclear program.

Asked if that figure was enough to make it worth while to the Iranian side, Cohen told Brennan she was right to point out that the relief on offer is, "actually quite modest, it's limited, and it's reversible."

It is so very deliberately, because so many skeptics -- including the Israelis and many in the U.S. Congress -- simply feel it's too soon to reduce the pressure on Tehran.

Seeking to ease those fears, Cohen told Brennan that he and his colleagues were going to travel the world and drive home the message that "anybody who acts in a way that violates any of the remaining sanctions will be dealt with."

Brennan said that if the talks in Geneva -- which are likely to go well into the weekend -- don't end in an agreement, the U.S. is ready to ratchet up the pressure with yet another round of sanctions.