SEYDA ZEINAB, Syria—The Lebanese movement Hezbollah, facing a heavy strain on its resources, is recruiting more fighters in Syria and bringing in fresh but inexperienced forces from Lebanon to shore up Bashar al-Assad's regime.

In the past year, Hezbollah's battle-tested fighters helped Syrian forces retake territory around the capital Damascus and other key cities such as Homs and Aleppo, paving the way for Mr. Assad to win a third, seven-year term as president in elections last month.

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