The group of 11 countries - Britain, Egypt, France, Germany, Jordan, Italy, Qatar, Saudi Arabia, Turkey, the UAE and the US, dubbed as Friends of Syria - during talks with the opposition discussed ways to ensure that the Geneva II conference next month must be about a political transition in Syria away from the Assad regime.
UK Foreign Secretary William Hague, after the talks here, said, "We are as clear as he is that Assad has no role in a peaceful and democratic Syria."
Syria's moderate opposition should "commit itself fully" to planned peace talks, he said.
However, a defiant Assad said he intended to run for re-election in 2014.
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Planned talks in Geneva next month offered Syrians the "best hope to improve their lives", Hague was quoted as saying by BBC.
"The reason we have to make sure we are supporting and dealing with the moderate opposition committed to a democratic, pluralistic, non-sectarian future for Syria is precisely because if they don't have a role, then all the Syrian people have got left is a choice between Assad and extremists," he said.
Hague said the ministers had agreed to "put our united and collective weight behind the UN-led Geneva II process".
The Syrian opposition coalition had agreed to attend the London talks, saying it would focus on "these countries' understandings about Geneva II and what it should result in".
The Syrian National Council, a key member of the opposition's National Coalition, has opposed the Geneva talks as long as Assad remains in power.
US Secretary of State John Kerry earlier said that the Syrian opposition would never agree to Assad staying in power.