A week before the latest deadline to reach a nuclear deal between Iran and six world powers, an accord looks highly uncertain, with diplomats saying critical issues remain unresolved. Differences on Iran’s future nuclear-enrichment cap, the speed of sanctions relief, the timeline of an accord and future nuclear research make reaching even the outline of a deal unlikely by the Nov. 24 deadline. Diplomats involved said that could require a second extension of the talks, which have already passed a July 20 deadline. Negotiations resume Tuesday in Vienna, with U.S. Secretary of State John Kerry set to join Iranian Foreign Minister Javad Zarif in coming days as both sides test whether a compromise can be reached on the remaining, highly sensitive issues. The talks include Iran, the U.S., the U.K., France, Russia, China and Germany.