(CNN) -- Iran is rolling back parts of its nuclear program and getting relief from sanctions in return as an interim agreement aimed at gauging Tehran's willingness to curb its nuclear ambitions appears to be working with global powers gearing up for talks on Tuesday to forge a long-term pact.
"So far everyone, both Iran and all of the rest of us who provided some very limited, targeted sanctions relief have kept their commitments," Wendy Sherman, a senior State Department official and lead negotiator for the United States on the Iran deal, told Wolf Blitzer on Monday in an interview on CNN's "The Situation Room."
Sherman, the under secretary for political affairs, spoke from Vienna where talks on a comprehensive accord between Iran, the United States, Germany and other permanent members of the U.N. Security Council are due to begin on Tuesday.
The six-month deal was reached in November and took effect in January.
The Vienna talks "will build on that first step, because we don't want it to be the only step," Sherman said.
"We go into this negotiation very clear eyed, very sober," she said. "It's going to be very tough."
Her comments came as Ayatollah Ali Khamenei, Iran's supreme leader, said his country would not "renege" on the talks, but predicted they "will not lead anywhere."
"What I care about is what Iran does much more than what Iran says," Sherman said.
She added that any final deal will be contingent on Iran taking "concrete" verifiable steps that prevent it from ever obtaining a nuclear weapon.
The United States and its allies believe Iran is aiming to develop a nuclear weapon, while Tehran has said it's atomic efforts are peaceful.
Sherman said she had seen recent Iranian reports that Russia has offered to build a nuclear reactor inside Iran in exchange for oil shipments, but did not offer a comment on whether or not that was a good idea.
North Korea camps
Sherman, who served as a top official on North Korea during the Clinton administration, also spoke of the "horror" of Pyongyang's labor and prison camp system on the same day the United Nations released a report about the secretive country's dismal record on human rights.
"We know a whole generation of children is growing up (in North Korea) with stunted growth and stunted intellect," Sherman said with many of its people subsisting on twigs and plants due to a chronic food shortage.
"As I have always said North Korea is not a place any of us would want to live in, and I give the United Nations credit for highlighting the horrible human rights abuses that are going on in North Korea."
Sherman added the United States is involved in "constant efforts" to free Kenneth Bae, an American currently being held in North Korea who was said to have recently been moved to a labor camp.
Bae is "not well and needs to be released without precondition," she said.