Foreign Secretary William Hague has urged moderate opponents of Syrian President Bashar Assad to come together ahead of planned peace talks aimed at ending the brutal civil war.
Foreign ministers from 11 Western and Arab powers are meeting in London today to discuss how they can support the moderate opposition in the run-up to next month's peace conference in Geneva, Switzerland.
The main Western-backed opposition umbrella group, the Syrian National Coalition, is scheduled to meet in the coming weeks to decide whether to take part in the Geneva summit.
One of the most prominent factions within the coalition, the Syrian National Council, has said it has no faith in negotiations with the Assad regime and will not be part of the Geneva process.
However Mr Hague warned that the longer the conflict went on, the more the position of the extremist groups fighting Assad's forces would be strengthened.
"Syrians on all sides now need to make the effort and make the compromises necessary for a peace process to work," Mr Hague told the BBC Radio 4 Today programme.
"The longer this conflict goes on, the more sectarian it becomes, the more extremists are able to take hold. That is why we are making this renewed effort to get a Geneva peace process going.
"I am in no way glossing over or minimising the danger of extremism taking hold. There are people fighting for extreme groups, not necessarily because of extreme views, but because that gives them access to weapons and training and so on - all the more reason why we have to help the moderate opposition in Syria."
Mr Hague also raised the prospect that Iran - one of Assad's few remaining allies in the region - could take part in the Geneva talks, as long as it was prepared to accept the need for political transition.
"It is important that Iran plays a more constructive role. I have discussed the situation in Syria with the new Iranian foreign minister," Mr Hague said.
"If they are serious, of course it is helpful to have all outside powers involved in this, but that depends on them coming on a common basis of trying to achieve a peaceful settlement on the basis of a transitional government in Syria.
"That's the position of Russia and it's the position of all of us among the West and among Arab nations.
"It should be the position of Iran as well."
The countries in the "London 11", the core group of the Friends of Syria, are also trying to shift the focus back to the humanitarian crisis after weeks of international attention on Assad's chemical weapons stockpile.
Representatives of the Syrian National Coalition's leadership are attending, along with ministers from the United States, France, Germany, Italy, Qatar, Saudi Arabia, Turkey, Jordan, the United Arab Emirates and Egypt.