BEIRUT, Lebanon — The leader of the powerful Lebanese militant group Hezbollah decisively committed his followers on Saturday to an all-out battle in Syria to defeat the rebellion against President Bashar al-Assad. He said the organization, founded to defend Lebanon and fight Israel, was entering “a completely new phase,” sending troops abroad to protect its interests.

“It is our battle, and we are up to it,” the leader, Hassan Nasrallah, declared in his most direct embrace yet of a fight in Syria that Hezbollah can no longer hide, now that dozens of its fighters have fallen in and around the strategic Syrian town of Qusayr. Outgunned Syrian rebels have held on for a week there against a frontal assault by Hezbollah and Syrian forces.

The speech signaled a significant escalation in Hezbollah’s military involvement in Syria, deeply enmeshing the group in the war across the border.

It could put new pressure on the Obama administration and on Europe, where more countries have begun pushing to list the group as a terrorist organization as the United States does. It was also likely to further inflame tensions in Lebanon, where Syria’s civil war has spilled over in explosions of sectarian violence.