Hague was speaking before a meeting in London on Tuesday of the so-called “London 11” nations seeking to bolster the Syrian opposition and lay the groundwork for “Geneva 2”.
The talks face great obstacles, including divisions within the opposition, rivalry between rebel groups and President Bashar Al-Assad’s reluctance to give up power. Many of the mostly Islamist rebel factions fighting on the ground do not recognize the exile opposition backed by the West.
Hague said the opposition, elements of which are loathe to negotiate with Assad’s government, should attend the talks.
“If they are not part of a peace process in Syria then all the Syrian people have got left is to choose between Assad on the one hand and extremists,” he told BBC Radio.
“The longer this conflict goes on the more sectarian it becomes and the more extremists are able to take hold and that is why we are making this renewed effort to get a Geneva peace process going.”
US Secretary of State John Kerry told reporters events may have moved in Al-Assad’s favor since he and Russian Foreign Minister Sergei Lavrov announced plans for the peace conference in May, but both he and Hague sought to play this down.
“Neither side is winning this conflict militarily, neither is able to conquer the other,” Hague said.
“We have to see it in the longer term. Syrians on all sides now need to make the effort and the compromises necessary for a peace process to work and the appetite is there among the outside powers, in the rest of the world.”
Several officials, including Arab League chief Nabil Elaraby, have said they expect the Geneva 2 conference to convene on November 23, though the United States, Russia and the United Nations have all said no date has been officially set.
Britain, Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey, the United Arab Emirates and the United States will on Tuesday discuss the agenda for the meeting and how to help the opposition prepare for it, a US official said.
While Washington has said it is open to the possibility of Iran, which has supported Assad, coming to a Geneva conference, Kerry said it was hard to see Tehran playing a constructive role unless it backs the idea of a transitional government.
Hague said Iran must support a proposed interim government in Syria including figures from Assad’s administration and the opposition as the way to political dialogue and free elections.
“If Iran could start from that position as well as the rest of the us, then Iran would be more easily included in international discussions on the subject,” he said.