BEIRUT—Rebels in Syria said they had seized a military base in the northwest, the latest victory for a coalition of antiregime forces moving closer to President Bashar al-Assad’s strongholds.

The capture of al-Qarmeed base on Monday comes two days after rebel factions—including American-backed fighters, Islamist groups and al Qaeda’s Syrian arm—loosely coordinated to seize the strategic town of Jisr al-Shoghour. Both are in Idlib province, which borders Turkey.

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