Iraqi security forces are probably incapable of retaking large stretches of territory seized by Sunni insurgents in recent weeks without outside help, the Pentagon’s top leaders said Thursday as they sketched a bleak assessment of turmoil in the country and forecast a protracted conflict that would be difficult to contain.

Defense Secretary Chuck Hagel and Army Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, said at a news conference that U.S. commanders are still considering what potential military courses of action they will recommend to the White House and whether U.S. troops should take a more active role. But they said deeper U.S. involvement would hinge on Iraq’s ability to overcome deep-seated political and sectarian fissures and form a national-unity government.

Dempsey said the Iraqi army had “stiffened” its resistance to a fast-moving insurgency led by the Sunni fighters of the Islamic State of Iraq and Syria, which has renamed itself Islamic State and declared a caliphate on land it controls. He said government forces were “capable of defending Baghdad” but added that they would be challenged to go on the offensive without external support. […]