Iran, the United States, Britain, France, Germany, Russia and China reached a tentative framework for a nuclear pact on April 2 but several issues remain unresolved. They have a self-imposed June 30 deadline to arrive at a comprehensive agreement.
"If the other side respects what has been agreed in Lausanne and tries to draft, based on mutual respect, a comprehensive agreement with Iran that is sustainable..., then we can meet any deadline," Zarif said after meeting his Greek counterpart.
"If people insist on excessive demands, on renegotiation, then it will be difficult to envisage an agreement even without a deadline," he said in Athens.
France warned on Wednesday it was ready to block a final breakthrough deal unless Tehran provided UN nuclear inspectors access to all installations, including military bases.
Iran's supreme leader, Ayatollah Ali Khamenei, last week ruled out international inspection of Iran's military sites or access to nuclear scientists under any agreement. Iran's military commanders echoed his remarks.