Days after Islamist militants swarmed the northern Iraqi city of Mosul, about 500,000 residents -- including Iraqi troops -- are exiting the city. 

The BBC describes a frightening scene: 

Residents of Mosul - Iraq's second city - said jihadist flags were flying from buildings and that the militants had announced over loudspeakers they had "come to liberate" the city. "The situation is chaotic inside the city, and there is nobody to help us," said government worker Umm Karam. "We are afraid." Many police stations were reported to have been set on fire and hundreds of detainees set free... The BBC's Jim Muir says the Iraqi security forces appear to have nothing to fight back against the ISIS militants.

The siege was accomplished swiftly, with members of the al Qaeda offshoot known as the Islamic State of Iraq and Syria (ISIS) taking control on Monday night after weekend clashes. The militants also ransacked the city's banks:

Now, reports are emerging that ISIS has also taken control of Baiji, an oil refinery town where weapons are stored: 

The group -- which is too radical even for its parent -- has been fighting alongside Sunni militants in Syria, gaining experience in the neighboring country before making moves in Iraq. The Telegraph reports that the steady rise of ISIS in Iraq has been underplayed because of the crisis in Syria, and describes how it gathered strength: 

[ISIS was] able to use both the human and financial resources for which the Syrian "cause" has been been a successful advertisement, particularly from religious sympathizers in the Gulf. More than in Syria, ISIS in Iraq took care not to alienate local populations. Some of the Sunni tribes who turned on al-Qaeda when it was fighting the Americans eight years ago were by now deeply hostile to the Shia-led government of Nouri al-Maliki. 

Mosul. REUTERS/Stringer

Iraqi leaders have harsh words for members of the military fleeing their posts in Mosul. "The (Iraqi) forces abandoned their weapons and the commanders fled, leaving behind weapons, armored vehicles -- their positions were easy prey for terrorists," said the country's parliamentary speaker of the situation in Mosul. Prime Minister Nuri al-Maliki added on Wednesday that all military leaders who left the city will be court-martialed.  

The government said today that it will work together with Kurdish forces to try to regain control in the area.