British Foreign Secretary William Hague says that Western and Arab diplomats have agreed that President Bashar al-Assad could play no role in any future government.
Hague was speaking after foreign ministers from the Friends of Syria group met with Syrian opposition leaders in London on October 22 to persuade them to attend a major peace conference in Geneva next month.
"We agreed that we would put our united and collective weight behind the UN-led Geneva II [peace conference] process, which must lead to establishing by mutual consent a transitional governing body with full executive powers," Hague said. "That is set out in Geneva Communique of June 2012. By definition, 'mutual consent' means that it can only be agreed with the consent of the Syrian National Coalition, so [Syrian President Bashar] al-Assad would play no role in that future government of Syria."
The Syrian National Council (SNC) -- a key group in the opposition's umbrella National Coalition -- has already refused to attend the conference, dubbed Geneva 2, which has been tentatively scheduled for November 23.
The SNC has also demanded President Bashar al-Assad step down as part of any agreement.
Hague urged the National Coalition to "commit itself fully" to the talks.
"Despite the enormous challenges faced by the Syrian opposition, we urge the National Coalition to commit itself fully to the Geneva II process and to lead and form the heart of any opposition delegation," Hague said. "Geneva offers the Syrian people the best hope to improve their lives."
U.S. Secretary of State John Kerry said after the talks that it is "imperative" that both sides in the Syrian conflict come to the negotiating table, warning that the country otherwise faces "implosion."
Assad said on October 21 that he would run for reelection in 2014 and that the factors were not in place for the Geneva conference to succeed.
Kerry and Hague were joined at the London talks by counterparts from Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey, and the United Arab Emirates.
Syrian opposition representatives are due to meet separately in early November to thrash out their differences.
Meanwhile, Sigrid Kaag, the chief of the joint Organization for the Prohibition of Chemical Weapons (OPCW) and United Nations mission said that Damascus has so far "fully cooperated" in destroying its chemical arsenal.
A team of UN-OPCW inspectors arrived in Syria on October 1 to oversee the implementation of a UN Security Council resolution that orders the destruction of Syria's chemical weapons and production facilities by mid-2014.
The Security Council passed its resolution after hundreds of people were killed in chemical attacks near Damascus on August 21.
Washington threatened military strikes but then reached a deal with Russia -- Syria's main international backer -- which became the basis for the Security Council resolution.
However, Kerry warned on October 22 that the sole elimination of Syria's chemical weapons arsenal will not bring to an end the country's devastating civil war.
Based on reporting by Reuters, AFP, dpa, and AP