By Mariam Karouny and Laila Bassam
BEIRUT (Reuters) - Hezbollah is fighting across all of Syria alongside the army of President Bashar al-Assad and is willing to increase its presence there when needed, the leader of the Lebanese Shi'ite movement said on Sunday.
Sayyed Hassan Nasrallah told thousands of supporters via video link that the fight was part of a wider strategy to prevent groups like al Qaeda's wing in Syria, Nusra Front, and the ultra-hardline Islamic State from taking over the region.
"Our presence will increase whenever it should... Yes, we are not present in one place in Syria and not the other. We will be everywhere in Syria," he said during a celebration to mark the withdrawal of Israeli soldiers from south Lebanon in 2000.
Hezbollah, backed by Iran, is a staunch ally of Assad in the four-year-long Syrian civil war. The conflict has become a focal point for the struggle between Tehran and Sunni Saudi Arabia, which has backed the insurgency.
Nasrallah also said that an offensive his group is leading in the mountainous region of Qalamoun along the border between Syria and Lebanon will last "until the borders are secured."
He said the residents of the area "will not accept the presence of terrorists and takfiris in any of the Bekaa or Arsal outskirts." Takfiri is a term for a hardline Sunni Muslim who sees other Muslims as infidels, often as a justification for fighting them.
Lebanon suffered its own civil war from 1975 to 1990, and officials there have warned Hezbollah against launching a cross-border attack which they say would drag the country further into the Syrian conflict.
Some also fear Hezbollah's offensive might provoke Sunnis in Arsal, a Lebanese town whose people have sympathized with the revolt against Assad and have welcomed thousands of Syrian refugees in the past four years.
Insurgents have tried to use the town as a base, and Nusra and Islamic State briefly seized it last year. They captured dozens of Lebanese soldiers and police and took them with them when they pulled out, later beheading and shooting four of them.
(Reporting by Mariam Karouny; Editing by Mark Trevelyan)