1,000,000 Iraqis have fled their homes as ISIS continues armed siege

More than a million Iraqis have been forced from their homes by conflict this year, the U.N. refugee agency said Friday -- a number only likely 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.

Concern for the soaring number of people displaced in Iraq comes as the UNHCR marks World Refugee Day on Friday.  Its report, based on data up to the end of 2013, finds that the number of refugees, asylum-seekers and internally displaced people worldwide has, for the first time since World War II, exceeded 50 million people.

Now the crisis in Iraq -- which was already sheltering hundreds of thousands displaced by the war in Syria -- is swelling that number even further.

An estimated 500,000 people fled Iraq's second city, Mosul, last week after it fell to fighters from the Islamic State in Syria and Iraq.

Already, a half-million were displaced from Iraq's western Anbar province, where the Sunni militants have been dominant since early this year.

Added to that are tens of thousands more who have fled their homes in Diyala and Salaheddin governorates amid the recent violence, UNHCR spokeswoman Ariane Rummery told CNN.

Will the number soar still higher? "In terms of the number of displaced people, that really depends on what happens," Rummery said. "But what we know is that conflict and violence fuel displacement.

"With the humanitarian situation, we know that there's a million people displaced and it's not always

easy to get to all of them."

While some fleeing families have taken rooms in overcrowded hotels, others, low on funds, have had to take shelter in transit camps that have sprung up near checkpoints controlling entry to Iraq's Kurdish-controlled north.

Those who have fled to the Kurdish region are easier to reach, Rummery said, but others have sought safety in less accessible regions.

U.N. High Commissioner For Refugees Antonio Guterres, speaking in Beirut, warned that any massive flow of refugees out of Iraq could be hard to cope with.

"Looking at the region, one asks ourselves, how can refugees from Iraq be received in the context of all the other situations that exist around Iraq? So, I hope that this outflow will never come because the capacity of the region to deal with it is practically nonexistent."

U.S. sending military advisers

For days, the United States has considered what to do about the militants, and on Thursday, U.S. President Barack Obama said he is prepared to send aas many as 300 military advisers to Iraq, adding that America was not returning to a combat role in the country.

The Pentagon earlier presented Obama with a plan to send up to 100 special forces advisers to Iraq to work along with that country's military, several U.S. officials told CNN on Thursday.

The U.S. withdrew its final troops from Iraq at the end of 2011, nearly nine years after leading the invasion of Iraq that toppled longtime leader Saddam Hussein.

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 Shia-dominated government is accused of fostering sectarian tensions by marginalizing Iraq's Sunni Arab and Kurd minorities.

The officials, along with Arab diplomats, say the White House is now focused on a political transition that would move Iraqis toward a more inclusive government -- one without al-Maliki that would include Sunni, Shiite and Kurdish factions.

Newly recruited Iraqi volunteer fighters take part in a training session in Karbala on June 17.

Top cleric urges all Iraqis to unite against ISIS

Al-Maliki has called for Iraqis to join the military's fight against the Sunni extremists -- a call echoed a week ago in a message from Ayatollah Ali Sistani, the most powerful Shiite cleric in Iraq.

But a senior religious Shia cleric clarified Friday that the top Iraqi Shiite leader was calling for all sects to stand together against ISIS, and not for the country's Shiites to take up arms in a new militia force.

The government should ban all militias and armed groups other than the Iraq Security Forces, Imam Sayid Ahmed Al-Safi said at Friday prayers in Karbala.

Additionally, he said Iraqis should respect the timeline set by the high court to name the new presidential cabinet and ministerial positions, following recent elections. Al-Maliki's party came out on top in the ballot but many in Iraq do not want to see him continue as Prime Minister.

Al-Safi also urged Iraqis not to engage in price-gouging. The cost of essentials such as food and cooking gas has soared in recent days, adding to people's anger.

Iraq's military is also calling for retired aircraft technicians, mechanics and officers to re-enroll for active service, the Ministry of Defense announced Friday. They will get their old titles and salaries back.

Precious seats on flights

The ISIS fighters, who have vowed to take the capital, have come within 40 miles of Baghdad with their assault on the town of Baquba.

The sense of fear was palpable Thursday at Baghdad International Airport, where hundreds of people waited in long security and check-in lines for one of the few, precious seats available on flights out of the capital.

Many people are seeking safety in Iraq's Kurdish-controlled north, particularly Irbil, or in the southern port city of Basra. Others are trying to quit the country altogether.

To accommodate the exodus, airlines have begun adding flights and, in some cases, much larger planes. Still, there are few seats available as most flights, according to travel agencies, are sold out weeks in advance.

Where once there was one Iraqi Airways flight a day to the northern city of Irbil, there are now three.

Amid growing unease, Australia's Defense Department said a small military contingent had been deployed to help protect the Australian Embassy in Baghdad.

Oil refinery

Fighting has focused this week on Baiji, where Iraq's largest oil refinery is located.

Neither side is in control of the complex, which refines much of the fuel needed for internal consumption. There are already long lines at many gas stations across the country.

Fighting was also ongoing Friday in the northwestern city of Tal Afar.

Also on Thursday, ISIS militants took control of a facility 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. But "the materials pose serious health hazards to anyone attempting to access the bunkers."