This is the final day for talks aimed at getting Iran to back off its nuclear program, and several “significant differences” remain to be solved before a preliminary agreement.  And one of the main players has left the negotiations.

“We are working late into the night and obviously into tomorrow,” said US Secretary of State John Kerry in Lausanne, Switzerland on Monday night.  “There is a little more light there today, but there are still some tricky issues.  Everyone knows the meaning of (Tuesday).”

Diplomats from France, Britain, Germany, China, Russia, and the European Union are trying to get Tehran to give up or delay the program in exchange for lifting economic sanctions that are choking Iran’s economy.  But out of that group, Russian Foreign Minister Sergei Lavrov has left the negotiations, saying he’d return to Lausanne if there were a chance of making progress.

Among the issues, Iran wants all western economic sanctions to be lifted the moment a final deal is signed, while the US prefers a gradual easing of the pressure.  If Iran violates a deal in the future, the US wants the sanctions to kick back in automatically – Russia and China want it to go back to the UN Security Council, where they have veto power.

Another problem came up on Sunday when Iran announced it had no intention of sending its existing stocks of enriched Uranium to the US under any deal.