WASHINGTON—With a limited ability to gather intelligence in Syria, U.S. spies have been tapping an unlikely source for information on Islamic State militants: the communications of President Bashar al-Assad’s regime.

The U.S. initially set up its spy infrastructure to monitor the regime, but it is also yielding unexpected intelligence over the Sunni jihadists that has helped guide American military operations in Syria and Iraq, current and former U.S. officials with knowledge of the operations said.

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