UN condemns Iraqi militant attacks
The ISIS fighters swept south from Mosul through Baiji, before moving into Tikrit.
Baiji hosts the nation's largest oil refinery but that appears to still be in government hands.
US State Department spokeswoman Jen Psaki said on Wednesday that Washington was committed "to working with the Iraqi government and leaders across Iraq to support a unified approach against ISIL's continued aggression".
Iraq has been seeking American drones to root out the insurgents, a request that has so far been rebuffed by the Obama administration, the New York Times reports.
Iraqi PM Nouri Maliki has vowed to fight back against the insurgents and said that he would punish troops who fled offering little or no resistance.