Arab governments and Turkey on Monday urged Syrian opposition groups discussing a unity plan in Cairo to end divisions that have cast doubt upon their readiness to lead Syria, DPA reported.
The Arab League-sponsored unity talks were boycotted by the rebel Free Syrian Army (FSA), which is fighting to overthrow President Bashar al-Assad.
"The Syrian opposition has an opportunity today that should not be wasted," Arab League chief Nabil al-Arabi said. "The sacrifices of the Syrian people are bigger than any differences."
The Syrian National Council (SNC), the main opposition group, on Sunday rejected a plan agreed by world powers to end the 16-month crisis. It called for the formation of a unity government that includes the opposition as well as members of the al-Assad regime.
International envoy Kofi Annan, who convened the meeting in Geneva, said failure to act would turn the conflict into a regional or international crisis. A ceasefire brokered by Annan in April never took hold.
"A splintered opposition is in the interests of the other side," Turkish Foreign Minister Ahmet Davutoglu said, urging Syrian opposition groups to form a united front against al-Assad.
"You need to send a strong message that you are united," he said. "We will meet soon in Damascus."
Tensions between Syria and Turkey rose after the Syrian army last month downed a Turkish military jet it said entered its airspace. Turkey, which backs the rebels and has urged al-Assad to step down, denied that and said it was changing its military rules of engagement along their shared border.
SNC spokesman George Sabra told dpa that he expected the unity talks attended by 250 delegates to be successful and that a plan outlining a transition period for Syria would be announced.
The FSA said in a statement it was boycotting the unity talks because the opposition has failed to secure "implementation of buffer zones protected by the international community, enforcement of an air embargo and arming the rebels."
The rebels, which are believed to receive funding and weapons from Saudi Arabia and Qatar, have intensified their military campaign against government forces, bringing their struggle to the capital Damascus.
A Red Cross team entered the town of Douma on the outskirts of Damascus, which has for weeks been shelled and besieged by government forces. It lies some 15 kilometres north-east of Damascus.
"Our joint team entered (Douma) yesterday and will attempt to go there again today to bring urgent medical and humanitarian assistance," said Khaled Erksoussi of the Syrian Red Crescent.
Activists said 20 people were killed in violence on Monday, including 16 in the central province of Homs, where government forces are fighting to regain control of rebel-held areas.