Hezbollah leader Hassan Nasrallah said on Thursday his militant Shi'ite group, which has been fighting alongside President Bashar Assad's forces in Syria's civil war, would stay in Syria for as long as necessary.

"As long as the reasons (to fight in Syria) remain, our presence there will remain," Nasrallah said in a speech to tens of thousands of Lebanese Shi'ites marking the religious ceremony of Ashoura in southern Beirut.

"Our fighters are present on Syrian soil...to confront all the dangers it faces from the international, regional and takfiri attack on this country and region," Nasrallah said, referring to the foreign Islamist rebels fighting in Syria.

Syria's 2-1/2 year-old civil war has polarised the Middle East between Sunni Muslim powers such as Turkey and the Gulf Arab states who support the Sunni rebels, and Shi'i'te Iran and its Lebanese ally Hezbollah who back Assad, from the Alawite faith which is an offshoot of Shi'ite Islam.

Hezbollah fighters led the fight to recapture the Syrian border town of Qusair earlier this year and activists say they have also been fighting alongside Assad's forces south of the capital Damascus and in the northern city of Aleppo.