British Foreign Secretary William Hague said on Wednesday Iran and its militant Shi'ite Lebanese ally Hezbollah were "propping up" Syrian President Bashar al-Assad and giving him increasing support.
"It is very clear that Syrian regime is receiving a great deal of support, increasing support in recent months from outside Syria from Hezbollah and Iran. This is a regime that is increasingly dependent on external support," Hague said in a news conference with his Jordanian counterpart Nasser Judeh.
"The regime is being propped up by others outside, further undermining its legitimacy. It also shows that is a crisis that is increasing the threat to regional stability."
Speaking before a meeting of the Friends of Syria alliance in the Jordanian capital, Hague said Britain would urge world powers to set a date in the next few days for an international conference to try to end the two-year conflict.
"It is important that it takes place as quickly as possible because people are dying and more and more people are driven from their homes. The risk to regional stability grow every day so this something that cannot be debated endlessly," Hague said.
His comments echo those of an EU diplomatic source, speaking late on Tuesday.
"Since November the regime has been able to go on the offensive not because it is stronger, but because those backing it - Iran, Russia, Hezbollah - are directly helping it either through weapons, planning of operations or financial assistance," the source attending the meeting in Amman said.
Assad and his supporters would be mistaken if they believed they could resolve the crisis by a decisive military defeat of their opponents, Hague said. "They need a political solution whatever the circumstances on the ground ... Their country is collapsing beneath them," he added.
Britain and France meanwhile have been trying to persuade the European Union to amend sanctions on Syria to allow some weapons to be sent to rebel forces.
"We do want to amend the arms embargo on the (opposition) National Coalition in order to be able to ... have the legal right to provide ... increased assistance, including if we decided to do so, lethal assistance," Hague said.
"So the decision we need to take in Europe certainly within the next 10 days is about the rules of that embargo, decisions about sending lethal equipment."
Judeh struck an upbeat tone about the prospects of a political outcome of the conflict. "There has been progress on the road to a political solution," he said.