Officials have said that a long-awaited deal on curbing Iran's nuclear ambitions may be finally within reach, after years of fruitless talks were given fresh momentum by the election of Iran's new President Hassan Rouhani, seen as a moderate.
In their second meeting in Geneva in less than a month, negotiators from the United States and five other global powers sat down with Iranian officials for two days of talks aimed at hammering out an agreement.
Ashton started the day with a private meeting with Iranian Foreign Minister Mohammad Javad Zarif to work out how talks would proceed.
Both sides have said recent talks have been the most productive in years but admit that reaching a deal will not be easy, with Zarif saying this week's negotiations were expected to be "very difficult".
"We have entered a detailed phase that is still difficult and precise," Zarif said on his Facebook page.
The meeting is the second since Rouhani took office in August pledging to resolve the nuclear dispute and lift sanctions by engaging with world powers.
Iran is anxious for relief from crippling economic sanctions that have cut oil revenues in half, caused the value of the rial to plunge and pushed inflation above 40 percent.
The West is also keen to seize a rare opportunity to build bridges with Iran after decades of hostility, opening the door to engaging with Tehran on other issues like the conflict in Syria, where Iran has backed President Bashar al-Assad against the anti-regime insurgents.