Baghdad, Iraq (CNN) -- More than 1 million Iraqis have been forced from their homes by conflict this year, the U.N. refugee agency said Friday -- a number likely only to rise as Islamist militants and Iraqi security forces battle for control.
A humanitarian crisis is brewing, as families who've fled fighting with little more than the clothes on their back seek water, food and shelter from the summer heat.
Meanwhile, the first of up to 300 U.S. military advisers will arrive in Iraq as soon as Saturday, a senior defense official told CNN. This first group from outside Iraq is expected to be very small, the official said.
In addition, some U.S. military personnel already in Iraq at the security cooperation office in the U.S. Embassy in Baghdad will be reassigned and become the first of the advisers to go to work, Pentagon Press Secretary Rear Adm. John Kirby said.
An estimated 500,000 people fled Iraq's second-largest city, Mosul, last week after it fell to fighters from the Islamic State in Syria and Iraq (ISIS). On Friday, the International Committee of the Red Cross in Iraq put the number who fled Mosul, with its population of 1.6 million, at about 800,000.
Already, a half-million people were displaced from Iraq's western Anbar province, where Sunni militants have been dominant since early this year.
U.S. sending military advisers
For days, the United States has considered what to do about the militants, and on Thursday, President Barack Obama said he was prepared to send as many as 300 military advisers to Iraq, adding that America was not returning to a combat role in the country.
The first group of advisers will begin work by conducting an initial assessment of Iraqi troop capabilities and on what may be needed for a larger group of U.S. advisers, including additional security measures where they may be deployed, a senior defense official said Friday.
But the United States has not reached an agreement with Iraq to provide legal protections to the U.S. military advisers.
"We are pursuing something in writing," Kirby said Friday. He said Defense Secretary Chuck Hagel "is absolutely committed to making sure that our troops have the legal protections, and he would not do that on a nod and a wink."
Kirby said the U.S. did not foresee a problem getting the Iraqi government to sign such an agreement because Iraq had requested this U.S. support.
The United States withdrew its final troops from Iraq at the end of 2011, nearly nine years after leading the invasion that ousted longtime leader Saddam Hussein.
Al-Maliki under pressure
As ISIS, born from an al Qaeda splinter group and supported by many Sunni factions, continues its fierce advance in Iraq, senior U.S. officials tell CNN that the Obama administration is of the belief that Prime Minister Nuri al-Maliki is not the leader Iraq needs to unify the country and end sectarian tensions.
The Prime Minister's Shiite-dominated government is accused of fostering sectarian tensions by marginalizing Iraq's Sunni Arab and Kurd minorities.
Obama told CNN on Friday that U.S. military efforts are hopeless without a change in government.
"If we don't see Sunni, Shia and Kurd representation in the military command structure, if we don't see Sunni, Shia and Kurd political support for what we're doing, we won't do it," he told CNN's Kate Bolduan in an interview.
The complete interview will be aired Monday on CNN's "New Day."
ISIS takes old chemical facility
Fighting raged across Iraq. Iraqi security forces regained control of the Baiji oil refinery, the largest in Iraq, on Friday following a night of fighting, Iraqi security officials in Samarra told CNN.
Iraqiya State TV reported that Iraqi security forces killed an undisclosed number of ISIS fighters.
Also on Thursday, ISIS militants took control of a facility that Saddam Hussein once used to produce and store chemical weapons.
But the State Department doubts that the Al Muthanna complex contains any material of "military value."
"The materials in the bunkers, which date from the 1980s, are of little military value and would be very difficult to safely move," State Department spokeswoman Jen Psaki said Thursday.
CNN's Chelsea J. Carter reported from Iraq and Laura Smith-Spark reported and wrote in London. CNN's Nic Robertson in Baghdad, Mohammed Tawfeeq, Khushbu Shah, John Raedler, Sherko Raof and Michael Martinez contributed to this report. Reza Sayah contributed from Tehran.