The fresh gains by insurgents, spearheaded by fighters from the al-Qaida-inspired Islamic State of Iraq and the Levant, come as Prime Minister Nouri al-Maliki's Shiite-led government struggles to form a coherent response after militants overran the country's second-largest city of Mosul, Saddam Hussein's hometown of Tikrit and smaller communities, as well as military and police bases — often after meeting little resistance from state security forces.

The fast-moving rebellion, which also draws support from former Saddam-era figures and other disaffected Sunnis, has emerged as the biggest threat to Iraq's stability since the U.S. withdrawal at the end of 2011. It has pushed the nation closer to a precipice that could partition it into Sunni, Shiite and Kurdish zones.