VIENNA- The US and Iranian foreign ministers will meet on Saturday, a senior US official said, as major differences persist over an agreement under which Iran would curb its nuclear program in exchange for sanctions relief.US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif will sit down in the Austrian capital ahead of Tuesday's self-imposed deadline, which many officials expect to slip.Significant gaps remain, notably over the sequencing of economic sanctions relief for Iran and the nature of monitoring mechanisms to ensure Tehran does not cheat on any agreement."The next few days will be extremely difficult," a senior Western diplomat said on condition of anonymity, adding that the talks may slide at least two or three days past the deadline.Senior Iranian negotiator Abbas Araqchi told Iranian reporters that the current negotiations were a "slow and difficult process."In addition to Iran and the United States, the talks include Britain, China, France, Germany and Russia. Foreign ministers from all the nations are expected in Vienna in the coming days to "check in" on the progress of negotiations.Officials close to the talks say they have yet to agree on the speed and scope of lifting sanctions, how Iran will reduce its stockpiles of low-enriched uranium, the future extent of Tehran's enrichment-centrifuge research and development program, and access for United Nations inspectors to military and other sites, as well as UN access to Iranian nuclear scientists. Another outstanding issue is what Iran may be required to do to address questions about the potential military dimensions (PMD) of its past nuclear work.Iran wants sanctions lifted immediately, though diplomats say they will be eased gradually in accordance with a schedule and only after confirmation that Iran has met its commitments to curb its nuclear program.Iran rejects allegations from Western powers and their allies that it is seeking the capability to produce nuclear weapons under cover of a civilian atomic energy program.