Influential Sunni clerics from several Arab states including Saudi Arabia and Egypt called on Thursday for a holy war against the "sectarian" regime in Syria.
"We must undertake jihad to help our brothers in Syria by sending them money and arms, and providing all aid to save the Syrian people from this sectarian regime," they said in a statement at the end of a gathering in Cairo.
"The flagrant aggression of the Iranian regime, of Hezbollah and of their sectarian allies in Syria amounts to a declaration of war against Islam and Muslims."
Lebanon's Iran-backed Shiite movement has been fighting alongside the forces of President Bashar Al-Assad, a member of the Alawite offshoot of Shia Islam, against Syria's mainly Sunni rebels.
Sunni-dominated Saudi Arabia's top cleric Abdulaziz al-Shaikh has urged governments to punish the "repulsive sectarian group" while Qatar-based Sunni cleric Yusuf al-Qaradawi has called on Sunnis to join the rebels.
Hezbollah's intervention in Syria, which helped Assad's troops overrun the strategic town of Qusayr, has been roundly condemned by Arab countries.
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In Cairo, a senior aide to President Mohamed Morsi demanded the guerilla group "immediately end" its involvement in Syria.
The Shiite group's assistance to Assad could "further turn this conflict into a sectarian conflict that will spill over into the entire region," Khaled Al-Qazzaz told reporters on Thursday.
The conflict has drawn Sunni volunteers from several Arab countries to join rebel ranks, and Shiites from Iraq who support Assad.
Qazzaz, Morsi's secretary on foreign relations, said the government was not trying to stop Egyptians from volunteering in Syria, mostly in relief work.
"The right of travel or the freedom of travel or taking certain positions is open for all Egyptians," he told reporters at a briefing.
"But we did not call on Egyptians to go and fight in Syria," he said.
Egypt believes the conflict will have to be resolved politically, he added.