DERIK, Syria—U.S. jets, drones and bombers pounded Sunni insurgent positions to ease the siege of the strategically vital Mosul Dam, as Washington and its Kurdish allies turned up pressure on the radical group Islamic State.

(James Foley: Islamic State claims killing of American journalist).

The militants retreated from some of their positions around the dam, the latest front across Iraq where Kurds have gained in recent days with the aid of stepped-up U.S. air attacks, advisers and weapons, and a controversial new ally: fighters from a Kurdish guerrilla force that Washington considers a terror organization.

Hundreds of guerrillas linked to the Kurdistan Workers' Party, or PKK, have this weekend fought in a broader Kurdish offensive against the insurgents under U.S. air cover. They joined the semiautonomous Iraqi Kurdish region's Peshmerga forces around the regional capital of Erbil and the Sinjar mountains, where thousands from the Yazidi religious minority have been trapped by the rapid advance of Islamic State fighters.

(Iraq: U.S. airstrike success spurs push for more attacks).