Major powers urged Syrian President Bashar al-Assad to commit to peace and step down as they held talks in neighbouring Jordan on Wednesday on preparations for a Russian-US proposed peace conference.
Ahead of the meeting of the Friends of Syria group in Amman, US Secretary of State John Kerry urged Assad to make a "commitment to find peace" after more than two years of conflict that have killed more than 90,000 people.
But he said that "in the event that we can't find that way forward, in the event that the Assad regime is unwilling to negotiate... in good faith, we will also talk about our continued support and our growing support for the opposition to permit them to continue to be able to fight for the freedom of their country."
Britain and Qatar urged Assad to step down.
"It is the longstanding view of the UK that Assad needs to go, and we have never been able to see any solution which involves him staying," British Foreign Secretary William Hague told reporters.
Qatar, a key supporter of the Syrian opposition, echoed that.
"A political solution must be reached to end the conflict and meet the aspirations of the Syrian people who, as we know, demand changing the regime and changing President Bashar al-Assad, who insists on killing his people," Sheikh Hamad bin Jassem al-Thani said in Doha.
The foreign ministers of Britain, Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey, the United Arab Emirates and the United States attended the meeting in the Jordanian capital.
Officials close to the talks said the ministers had met for more than two hours in a hotel in Amman, before meeting behind closed doors with the Syrian opposition.
The United States and Russia, which back opposite sides in the conflict, earlier this month proposed a peace conference dubbed Geneva 2 to bring together rebels and representatives of Assad's regime.
The aim of the conference, Hague stressed, would be to agree on the formation of "a transitional government with full executive authority, formed on the basis of mutual consent."
French Foreign Minister Laurent Fabius agreed.
"There are some conditions and in particular conditions about participation, which must be representative and which must not include countries which are against success," he told reporters in Amman, in an apparent allusion to Assad ally Iran.
Russian Foreign Minister Sergei Lavrov meanwhile hailed the Assad regime's "constructive reaction" to the conference as he welcomed Syrian Deputy Foreign Minister Faisal Muqdad in Moscow.
But Lavrov said the initiative was being "undermined" by the actions of the opposition in Syria.
While Damascus has reportedly already proposed the names of several potential envoys to the mooted Geneva conference, the opposition has yet to decide whether it will attend.
The National Coalition is to meet later this week in Istanbul, where it is expected to hammer out its stance on the peace effort and take decisions on its leadership.
The diplomatic drive came as Syrian regime forces and their allies pushed to retake the rebel stronghold of Qusayr in central Homs province bordering Lebanon.
The battle is drawing in neighbouring Lebanon, with the country's powerful Shiite movement Hezbollah dispatching fighters to bolster regime troops.
The Syrian opposition urged fighters across the country to "rush to the rescue" of Qusayr and appealed to the international community to set up a humanitarian corridor to the embattled town.
Troops backed by Hezbollah fighters attacked the village of Hamadiyeh north of Qusayr on Wednesday, one of the last remaining rebel positions in the area, activists said.
"The Syrian regime is receiving help from Hezbollah and Iran. That's an increasing threat to regional stability," Hague told reporters.
"If the regime were to think they can win a military victory and goes back to whatever was normal before, I think they will be making a terrible error."
Rebels captured an army camp in the northwestern province of Idlib on Wednesday in fighting which killed 40 Syrian soldiers and pro-regime militiamen as well as 14 rebels, the Syrian Observatory for Human Rights said.
The battle for Qusayr has raised tensions in the Sunni-majority Lebanese city of Tripoli, home to a minority of Alawites, the Shiite offshoot to which Assad belongs.
Clashes between the rival communities have killed at least 11 people in the port city since Sunday.
On Tuesday, the Observatory said more than 100 people had been killed in the fighting in Qusayr since the assault started, including 31 Hezbollah fighters, 70 rebels and nine soldiers.