Syrian opposition forces kidnapped 12 Lebanese Shiites in northern Syria on Tuesday, sparking fears the 15-month old conflict could spill into other countries.

The Shiites were on their way back from a pilgrimage in Iran, when they were ambushed in Syria's Aleppo province, according to Hezbollah's Manar Television station, the AP reported.

The abduction has caused protests in Beirut, where the leader of Shiite militia Hezbollah warned against revenge attacks on Syrians.

This is strictly prohibited, Sheik Hassan Nasrallah said in a televised speech, according to the AP.

We will work day and night until these beloved people are with us, he added.

Hezbollah has been an ardent supporter of beleaguered Syrian President Bashar al-Assad, a member of the minority Shiite, Alawite sect.

The sectarian tension stirred up by the revolt in Syria has already spilled into Lebanon.

Last week, violence between supporters and opponents of Assad erupted in the Lebanese port city of Tripoli, leaving two dead and 20 injured.

The violence in Tripoli was sparked by the arrest of prominent Islamist Shadi Mawlawi by Lebanese security forces.

His supporters said his arrest was partly due to his association with Syrian refugees, who have flooded the city since the conflict began.