Difficult issues remain on the table as the world's most powerful diplomats meet in Switzerland with Iranian nuclear negotiators, Kerry told CNN on Monday.

"We are working very hard to work those through. We are working late into the night and obviously into tomorrow. We are working with a view to get something done," he said. "There is a little more light there today, but ​there are still some tricky issues. Everyone knows the meaning of tomorrow."

Negotiators have set Tuesday as their deadline for a basic deal. A comprehensive deal, including technical additions, is supposed to be negotiated by June 30.

World powers are seeking the outlines of an agreement they say would stop Iran from getting a nuclear weapon for at least 15 years. In exchange, Iran would get out from punishing sanctions that have crippled its economy.

Kerry's comments to CNN came after uncomfortable rumblings about the talks in Lausanne, Switzerland, made headlines.

The assertion: Iran backpedaled the day before on an important detail of a possible deal to prevent it from developing a nuclear bomb.

On Sunday, an Iranian negotiator told journalists that Tehran would not send fissile material to Russia, which diplomats had earlier told journalists was part of the plan to put potential bomb-making materials out of reach.

"The export of stocks of enriched uranium is not in our program, and we do not intend to send them abroad. ... There is no question of sending the stocks abroad," Iranian Deputy Foreign Minister Abbas Araghchi said.

But on Monday, a senior U.S. State Department official said the rumblings in the press should quiet down.

Negotiators had not yet decided any specifics about the disposal of fissile material, and Iran has made the comments many times before, the official said, citing a list of previous examples of such statements in press reports.

Chinese Foreign Minister Wang Yi sounded optimistic as he briefed reporters on the talks' progress earlier Monday, saying that the diplomats were "narrowing down" their differences and working out ways to resolve sticking points.

"These marathon-like negotiations have reached the final stage," he said.

Three tense points

Things have been tense in Lausanne as the deadline for an agreement looms, with talks snagged on three important points:

• How quickly or slowly Iran will be allowed to advance its nuclear technology in the last five years of the 15-year agreement.

• How quickly crushing U.N. sanctions will go away.

• Whether sanctions will snap back into place if Iran violates the deal.

(CNN)