On a windy hilltop overlooking the mountainous Syrian border, a fighter from Lebanon’s Hezbollah says the battle against militants in the area is among the toughest the group has ever faced.

The powerful Lebanese Shiite movement, a key ally of the government in Damascus, has fought across Syria in the years since an uprising began in 2011.

In the past two weeks, Hezbollah says it has secured around a third of the Qalamun region, territory on both the Lebanese and Syrian sides of the porous border. […]