"We have always been very clear that a peaceful solution in Syria must require the departure of President Assad," Hague told a press conference in Kuwait's capital after talks with his Kuwaiti counterpart Sheikh Sabah Khaled Al-Sabah.
"It is impossible to imagine after so many deaths, so much destruction, a regime oppressing and murdering its own people on this scale" should remain in power, Britain's chief diplomat said.
"It is impossible to imagine, I think, President Assad could remain on the scene in Syria in the future," Hague said.
The government in Damascus has said Assad will remain president and lead any transition agreed at the conference, while the opposition and rebels fighting the regime insist he play no role.