WASHINGTON—The U.S. is providing direct, covert military aid to Kurdish forces struggling to repel well-armed Sunni militants who have seized key parts of northern Iraq, U.S. officials said Monday.

The Central Intelligence Agency has been providing Kurdish fighters with ammunition and small arms, putting the spy agency at the vanguard of what the U.S. hopes will become an international effort. The Pentagon is similarly seeking ways to start its own direct supply efforts.

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