CAIRO — The Lebanese militant group Hezbollah and the Syrian Army launched a major operation on Friday that aimed to dislodge a pocket of Islamist fighters from their stronghold on the border between Lebanon and Syria.

Fighters and soldiers advanced on two fronts, near the Lebanese town of Arsal and the Syrian town of Fleita, into a mountainous area where an array of Islamist groups have taken shelter in recent years among camps for Syrian refugees.

Explosions rang out in eastern Lebanon as Hezbollah shelled a distant ridgeline while Syrian warplanes struck on the other side of the border. Early reports said both sides had casualties.

The offensive had been anticipated for weeks and comes at a time of mounting frustration over how the Syrian crisis has spilled over into Lebanon, a country of four million people that has become home to more than one million Syrian refugees since the uprising against President Bashar al-Assad erupted in 2011.