Main opposition group the Syrian National Coalition on Sunday expressed concern at jihadist Al-Nusra Front's pledge of fealty to Al-Qaeda, warning it would serve the goals of President Bashar al-Assad's regime.
"The Syrian Coalition is deeply concerned about recent statements regarding the affiliations and ideologies of particular factions of the rebel forces," the group said.
"Such initiatives only serve the goals of the Assad regime and harm the progress of the revolution," it added, calling on Al-Nusra "to stay within the ranks of nationalistic Syrians."
Al-Nusra has long been suspected of links to Al-Qaeda in Iraq, but the announcement of an alleged merger between the groups and its pledge to Al-Qaeda chief Ayman al-Zawahiri have raised hackles among some in the Syrian opposition.
Rebel fighters and activists have been reluctant to criticise the group publicly, unwilling to alienate one of the fiercest fighting forces battling the regime.
When the United States designated the group a "terrorist" organisation last December, the Syrian National Coalition and other opposition groups criticised the decision.
But there has been discomfort with both Al-Nusra's tactics, including suicide bombings, and its explicit calls for the establishment of an Islamic state in Syria, which Zawahiri also called for in an audio message last week.
"We stand against any forces that may inhibit the Syrian people's choosing of their own future," the Coalition said.
"The Syrian Coalition vows to support the Syrian people in their continued struggle for freedom and to stand against any forces that counter the noble objectives of the Syrian revolution."