Two days of exhaustive negotiations between U.S. Secretary of State John Kerry and his Iranian counterpart, Javad Zarif, in the Persian Gulf nation of Oman resulted in no significant breakthrough in forging a comprehensive agreement to curb Tehran’s nuclear program by a Nov. 24 diplomatic deadline, said senior U.S. and Iranian officials. “Real gaps” remain between Washington and Tehran, said an American diplomat, in describing talks that seek to end a decade-long standoff over the future of Iran’s nuclear program. Lower-level diplomacy between the U.S., Iran and other world powers continued on Tuesday in Muscat, as Mr. Kerry arrived in China to brief President Barack Obama on the status of the nuclear diplomacy.