UPDATE 3-U.S., Iran say disputes remain in nuclear talks as deadline looms

* Self-imposed July 20 deadline for deal looming * Big-power foreign ministers including Kerry join talks * Negotiations snag over Iran's nuclear capacity * Kerry, Steinmeier meet in Vienna after new spying accusations (Adds Steinmeier, Hague, Fabius comments, Kerry to meet Zarif) By John Irish and Lesley Wroughton VIENNA, July 13 (Reuters) - U.S. Secretary of State John Kerry said on Sunday ...