The Friends of Syria group, which backs the Syrian uprising, meets on Wednesday in Amman for a new round of talks expected to focus on a US-Russian bid to resolve the country's brutal conflict.
The group "will hold talks paving the way for the international conference on Syria in June, in order to reach a political solution to the Syria crisis", Jordan's foreign ministry said in a statement.
Eleven top diplomats from Britain, Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey, the United Arab Emirates and the United States, will be attending the Amman meeting.
Ahead of the gathering, US Secretary of State John Kerry will hold a joint news conference with his Jordanian counterpart Nasser Judeh.
Earlier this month the United States and Russia, which support opposite sides in the Syrian conflict, proposed a peace conference dubbed Geneva 2, which would bring together rebels and representatives of President Bashar al-Assad's regime.
For the first time in the 26-month conflict, the opposition, whose chief Ahmed Moaz al-Khatib resigned last month, has not been invited to the Friends of Syria meeting.
"The opposition has not been invited. I think this absence is because the Friends of Syria want to reach a unified position on the international conference," main opposition National Coalition member Anas Abdeh told AFP.
Their common stance "will be revealed at the opposition's meeting in Istanbul (Thursday), where they (Friends of Syria) will try and convince the opposition to take part in the (international) conference," Abdeh added.
In Turkey, the opposition is set to choose a new leader and to decide whether it will take part in Geneva 2.
A conference in Geneva in June 2012 called for an end to the violence ravaging Syria and laid the framework for a political transition, but stopped short of specifying whether Assad should stay or go.
And the main thorn in the negotiations over Geneva 2 is the question of Assad's fate.
The opposition says Assad and his regime's departure are a condition to any negotiations.
The Syrian president has frequently rejected the idea of stepping down, while referring systematically to both rebels and activists as "terrorists".
In a weekend interview with an Argentinian newspaper, Assad implied he would run for a new term in 2014.
More than 94,000 people have been killed in Syria's escalating war, which broke out after the regime unleashed a brutal crackdown against dissent, a watchdog says.
The international community is also split over the Syrian conflict.
Washington, Paris and London have led calls for Assad to resign, while Moscow has continued to support him and provide his regime with arms.
Even though Geneva 2 was proposed by the United States and Russia, the sharp divide over the Syria crisis has made talks leading up to the conference all the more complex.
Discussions have focused above all on who should take part in the conference, which aims to bring to the same table regime and opposition representatives.
France, meanwhile, refuses that Assad attend Geneva 2.
"Paris' position is clear: Assad is not part of the solution. As far as we're concerned, he cannot be in Geneva," a French diplomatic source told AFP on condition of anonymity.
The source also said that there needs to be an agreement that can be implemented on the ground.
"We don't want to be in a position where the opposition finds itself with its back against the wall, forced to sign up to unacceptable agreements," he added.