"Acceptance of the dictator's role, who wages a civil war against his own people, is not considered," Mogherini told the France Inter radio station, when asked about the prospects for Assad's political future.
At the same time, she underscored that the problem lies not only in the fate of one person.
"The question is how to start the transition of power, to try to change the management of the country, the constitution, the system of power, preserving the structure of the state, because there are concerns that the state will collapse," Mogherini added.
Earlier in April, the Syrian opposition High Negotiations Committee (HNC) postponed its participation in the latest round of the Syria peace talks in protest at the ongoing violence in Syria for which they blame Damascus. De Mistura said earlier on Friday that the talks would continue until April 27 as planned.