A series of U.S. air strikes since last week has slowed the operational tempo of the Islamic State, the militant group that has seized large parts of northern Iraq, but is unlikely to substantially weaken the group, the Pentagon said on Monday.

Holding a news conference, Lieutenant General William Mayville says strikes had temporarily disrupted the Islamic State's advance toward Arbil, the capital of Iraqi Kurdistan.

At least half of the 40,000 people besieged by jihadists on Mount Sinjar had escaped by Sunday night, aided by Kurdish rebels who crossed from Syria to rescue them.