With the militants closing in on the capital, forces from Iraq's autonomous Kurdish region took control of Kirkuk, an ethnically divided northern city they have sought to rule for decades against the objections of successive governments in Baghdad.
Iraqi Foreign Minister Hosyhar Zebari acknowledged the security forces which Washington invested billions in training and equipping before withdrawing its own troops in 2011, had simply melted away.
Obama said Iraq was going to need "more help from the United States and from the international community."
Russia said the lightning gains by the Islamic State of Iraq and the Levant (ISIL), a movement so radical it has been disavowed even by the Al-Qaeda leadership, showed the pointlessness of the 2003 US-led invasion, carried out in the aftermath of the 9/11 attacks in the United States.
Washington found rare common cause with its longtime foe Tehran, with both voicing dismay at the Sunni extremists' advance and pledging to boost aid to Iraq's beleaguered Shiite prime minister.
The militants, who have swept up a huge swathe of predominantly Sunni Arab territory in northern and north-central Iraq since launching their offensive in second city Mosul late on Monday, advanced into ethnically divided Diyala province.
ISIL spokesman Abu Mohammed al-Adnani vowed the jihadists would not stop there, but would press on to the capital and the Shiite shrine city of Karbala, visited by millions of pilgrims from around the world each year.
