Influential Muslim cleric Yusuf al-Qaradawi on Monday urged Al-Nusra Front to stay loyal to the mainstream Syrian rebel army, describing its pledge of fealty to Al-Qaeda as dangerous.
"Al-Nusra Front, which has done very well in its jihad (holy war) against the tyrant regime of Damascus, should ... remain within the ranks of the Free (Syrian) Army, to maintain unity," the International Union of Muslim Scholars which he heads said in a statement.
"This pledge causes internal and external dangers, and its impact on the revolution is dangerous, because it breaks the ranks of the mujahedeen," the Doha-based group said.
Listed by Washington as a "terrorist" group, Al-Nusra has been playing an effective role in the fight against President Bashar al-Assad's forces.
It 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 earlier this month have raised hackles among some in the Syrian opposition.
The union urged Al-Nusra Front "to leave the issue of rule and state in Syria until after the liberation, when the people can freely choose an Islamic system or any other system of rule."
It warned that the pledge to Al-Qaeda only "strengthens" the Assad regime.
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.