VIENNA—Iranian Foreign Minister Javad Zarif headed back to Tehran for what could be critical consultations Sunday evening as a senior U.S. official said all sides now expected nuclear talks between Iran and six major powers to go past the June 30 deadline.

A senior Iranian official confirmed Mr. Zarif was headed to Tehran and would return to the Austrian capital on Tuesday morning. He didn’t say who Mr. Zarif would meet back home, but Western officials said the trip could involve discussions among Iran’s leadership that may...