The Friends of Syria, an alliance of mainly Western and Gulf Arab countries who support the Syrian opposition, on Sunday called on them to attend peace talks in Switzerland later this month.
"We urge the National Coalition to respond positively to the invitation to set up the Syrian opposition delegation sent by the U.N. Secretary General," the group said in a joint statement.
"We invite them to form, as soon as possible, a delegation of opposition forces to participate in the political process."
They also said that the only way for political solution in Syria is for peace talks in Geneva to take place.
"There is no other political solution," French Foreign Minister Laurent Fabius said speaking on behalf of the group. "There will be no political solution for Syria unless Geneva 2 meets."
An international meeting bringing President Bashar al-Assad's government and opposition groups to the table is set to be held from Jan. 22 in Switzerland.
Meanwhile, activists say nearly 700 people have been killed in rebel infighting in opposition-held parts of northern Syria over the past nine days.
The fighting between the al-Qaeda-linked Islamic State of Iraq and the Levant and an array of Islamist and more moderate rebel factions is the most serious violence within the armed opposition since the Syrian conflict began.
The British-based Syrian Observatory for Human Rights activist group said Sunday that at least 697 people have been killed since the clashes began Jan. 3. The toll includes 351 fighters from the Islamist and mainstream opposition brigades, 246 from the "Islamic State," and 100 civilians.
The al-Qaeda-linked group has alienated other factions by using brutal tactics to implement its strict interpretation of Islamic law, and by kidnapping and killing of opponents.
On the field, Syrian activists said government shelling has killed more than 20 people in the central city of Homs.
The British-based Syrian Observatory for Human Rights said Sunday that the number is expected rise because dozens of people were critically wounded in the shelling of the rebel-held Waer area of Homs.
The Observatory said the shelling occurred Saturday. The activist collective, the Local Coordination Committees, also published similar information.
Forces loyal to Syrian President Bashar Assad have been widening an air assault on rebel-held areas in recent weeks, targeting opposition-held residential areas and killing civilians.
Syrian rebels have also targeted Assad-loyal areas with indiscriminate mortar fire.
On Sunday, Syria's state media said rebel-fired mortar shells killed two people in the Assad-loyal Zahra area of the city.
AP