Washington (CNN)World powers and Iran have made progress in closing some of the remaining gaps preventing a deal on Tehran's nuclear program, according to U.S. senior administration officials, but they warned Tuesday that difficult issues remain.
U.S. sees progress in Iran nuclear talks as deadline nears
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The sides met in in Lausanne, Switzerland, for a third day of grueling diplomatic negotiations that are gathering speed as an end-of-March deadline for a framework deal looms.
"We have definitely made progress" on technical issues — as opposed to other problem areas, such as sanctions — a U.S. official said. "There's no way around it."
But the official added that "we still have some tough, tough issues to address."
Tuesday's talks involved enrichment, reactors, fissile materials production and how all these factor into breakout times, among other topics, according to a senior administration official.
The negotiators also discussed transparency and verification, the official said.
If the parties are successful in hammering out a framework agreement this month, they will then have until July 1 to finalize all the details.
State Department Spokeswoman Jen Psaki described the discussions as "solid, substantive and difficult, but constructive."
She added, "We expect that will remain the case as we continue to try to close the gaps."
But time may be running out for a deal in which six world powers would lift sanctions that have throttled Iran's economy in return for assurances that Tehran will continue to stay a year or so away from developing a nuclear bomb.
The White House puts the chances of an agreement at only 50-50. Disputes still rage over the scale of nuclear infrastructure Iran will be allowed to keep, the pace of sanctions relief and the extent of nuclear site inspections.
Despite the controversy stoked last week in Washington when 47 GOP senators sent a letter to Iran's leaders warning that the future of a deal was not guaranteed without congressional support, many analysts believe that the talks will go on even if the end-of-March deadline slips.
Psaki called the letter "a distraction, and certainly a time suck," noting that the Iranians brought up concerns about its contents on the first day of this round of talks.
"We continue to believe that these negotiations are not about a letter that was ill-informed or ill-advised. They're about the issues at hand," Psaki said.
But beyond the GOP letter, Republican leaders in Congress are pushing measures that would give them a bigger role in a process they're highly critical of.
Senate Foreign Relations Committee Chairman Bob Corker, a Tennessee Republican, is working on legislation that would force the Obama administration to submit any deal to Congress for approval. The committee could vote on the bill as early as next week. The White House has threatened to veto it.
Meanwhile, Americans broadly back direct negotiations with Iran about that country's nuclear program, according to a new CNN/ORC poll.
Although about half (49%) said some Republican senators went too far with their letter to Iran's leaders, only about one-third (39%) thought
the letter hurt U.S. efforts to keep Iran from developing nuclear weapons.