WASHINGTON—The Central Intelligence Agency is ramping up support to elite Iraqi antiterrorism units to better fight al Qaeda affiliates, amid alarm in Washington about spillover from the civil war in neighboring Syria, according to U.S. officials.

The stepped-up mission expands a covert U.S. presence on the edges of the two-year-old Syrian conflict, at a time of American concerns about the growing power of extremists in the Syrian rebellion.

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