Iran and six world powers are launching a fresh effort to try to narrow stubborn differences on what nuclear concessions Tehran must agree to in exchange for full sanctions relief.

The talks, which resumed Friday at the United Nations, may be joined next week by U.S. Secretary of State John Kerry and his counterparts. The deadline for agreement is Nov. 24, putting pressure on both sides to seal a deal.

Iran's uranium enrichment program is the greatest stumbling block. Tehran says it is not interested in nuclear weapons and wants to enrich only to make reactor fuel. It seeks to keep its enrichment activities robust, but Washington wants deep cuts in the program because it can also be used to make the fissile core of a warhead.