"It's difficult. It's challenging. There's no guarantee we will succeed," Ashton told a joint press conference Sunday with Iranian Foreign Minister Mohammad Javad Zarif.
Ashton is meeting with top officials to discuss bilateral relations between the European Union and Iran, the Washington Post reported Sunday.
In November Iran and the P5-plus-1 countries -- the United States, Russia, China, Britain, France and Germany -- agreed to provide relief from sanctions to Iran in exchange for suspension of some of Iran's nuclear activities.
"Iran is determined to reach an agreement. We have shown good faith. We have shown political will. We have done our side," Zarif said.