Facing a midnight local time target to conclude a framework accord, substantial differences persisted with officials predicting a long day of talks that may or may not result in success.
The top diplomats of four of the five permanent members of the UN Security Council and Germany met alone and then with Iran's foreign minister to try to bridge the remaining gaps. They hope to hammer out an understanding that would serve as the basis for a final accord to be reached by the end of June.
"Long day ahead," the spokeswoman for the US State Department said in a tweet announcing the early morning start of the foreign ministers' meeting with Iranian officials.
Late yesterday, Secretary of State John Kerry told a CNN reporter that "everyone knows the meaning of tomorrow," adding that "there are still some tricky issues."
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Kerry and others at the table said the sides have made some progress, with Iran considering demands for further cuts to its uranium enrichment program but pushing back on how long it must limit technology it could use to make atomic arms. In addition to sticking points on research and development, differences remain on the timing and scope of sanctions removal, the officials said.
Iran's deputy foreign minister, Abbas Araghchi, told Iranian state television yesterday that the talks were not likely to reach any conclusion until "tomorrow or the day after tomorrow.