WASHINGTON—The U.S. dropped weapons, ammunition and medical supplies to Syrian Kurds fighting Islamic State extremists in the embattled city of Kobani, U.S. officials said Sunday.

Three U.S. C-130 cargo planes began dropping the weapons and supplies, provided by Kurdish authorities in Iraq, on Sunday, the officials said. Over several hours, the U.S. dropped 27 bundles of small arms, ammunition and supplies.

The...