— The United Nations children’s agency UNICEF on Wednesday upgraded Iraq’s latest crisis to a level 3 humanitarian disaster — its most severe designation — as U.N. officials said they were scrambling to provide basic services while preparing to cope with an estimated 1.5 million displaced people.

“Now we’re focused on delivering water, food and essential items,” said Colin MacInnes, deputy head of UNICEF in Iraq. The organization has been collaborating with other U.N. and humanitarian agencies to deliver aid.

The emergency distinction, which is rarely assigned, underscores just how critical Iraq’s crisis has become, one week after Sunni jihadists from the Islamic State of Iraq and Syria (ISIS) captured the northern Iraqi city of Mosul and began rapidly sweeping across the country, seizing towns and routing Iraqi forces.

“Iraq already has a level 3 polio disaster,” MacInnes said. The Syrian crisis next door is so desperate that it has also been named a level 3, he said. “With this designation in Iraq, that means we have currently three level-3 disasters that are affecting the country.”

He said he could not think of any other place or time in U.N. history where that has happened.

The battle between Islam’s two major branches began centuries ago and is threatening Iraq’s path to a stable democracy today. The Post’s senior national security correspondent Karen DeYoung explains. (Davin Coburn and Kate M. Tobey/The Washington Post)

According to the Kurdistan Regional Government in Irbil, in the past week, at least 300,000 displaced Iraqis have flooded into areas controlled by the KRG, north of Mosul. Kurdistan already hosts roughly a quarter-million Syrian refugees, said Falah Mustafa Bakir, the head of the KRG’s foreign relations department.

But humanitarian officials also say the situation is rapidly evolving, with displaced Iraqis moving between towns, into homes of relatives and friends, or from hotels into refugee camps when their money runs out.

Camilo Valderrama, the senior health adviser for the International Rescue Committee’s mission, said in one burgeoning camp north of Mosul, called Garmava, only three tents had been set up as of Saturday. When a reporter visited Tuesday, there already was a sprawl of white U.N. tents and more than 300 people, with more pouring in.

“We’re still counting,” said Ismail Abdelghani, a regional government official who is running the camp.

Nearby, Kurdish aid workers and refugees dug latrines, and a man in a bulldozer tried to even out the gravelly desert plain that the tents occupied.

“Tell them to bring us water,” said a taxi driver who identified himself only as Abdel Hady.

He said he had walked three days from Mosul with his eight-member family, sleeping in strangers’ homes. The sun beat down on the camp, turning the insides of the white U.N. tents — the only shade available — into saunas. Sweat ran down Abdel Hady’s face as he spoke.

Aid officials described Garmava, the closest camp to embattled Mosul, as a “last resort” — only the most desperate wind up in it. Many who showed up said they came at least part of the way on foot.

“There is nothing left for me. I don’t even own this rock,” said Taha, an Iraqi police officer from Mosul who gave only his first name, as he picked up a stone from the floor of his tent at Garmava. “If it was not for my children, I’d prefer to die.”

Taha said he had watched Mosul deteriorate in the months before its sudden fall to insurgents. He had received telephone death threats for being a police officer, and then someone blew up his house, he said. When ISIS overran Mosul, he fled north with his pregnant wife and four young children.

His next child is likely to be born homeless.

“I could give birth here any day,” said Taha’s wife, Shahla, as she squatted in the mud to fill a bucket with water at the edge of the camp, where hot wind whipped at the tents.

Other displaced Iraqis, from regions stretching from Tal Afar in the west to Diyala province in the east, are even closer to the danger zone because they were prevented from leaving by a lack of resources or by security concerns, humanitarian officials said.

On Tuesday, the United Nations delivered water and emergency aid to the town of Sinjar, one of the poorest in the country. Residents of Tal Afar fled to Sinjar this week after their ethnically mixed town was overrun by insurgents.

Fearing that polio will spread into Kurdistan along with the influx of refugees, UNICEF is planning a mass vaccination campaign next week.

But many of the displaced are still hard to track because they are moving around and are sheltering in homes, hospitals, schools and parks, rather than in camps, officials said.

Gabriel Tooma, the priest of the Chaldean Christian Church in the ancient Christian village of Alqosh, 28 miles north of Mosul, said his town is hosting about 1,200 people in homes, schools and administrative buildings — a number that is steadily growing.

It isn’t the first time that Alqosh, a village of stone houses nestled at the foot of a mountain, has taken in Iraqis fleeing sectarian violence in Mosul.

“I have one family that has fled here all four times,” Tooma said.

But Iraq’s ongoing crisis marks the first instance of exodus in which most — about 80 percent, he said — of the refugees arriving in Alqosh are Muslim. Many of them are Shiites from Iraq’s long-persecuted Turkmen minority.

The Christians are fewer — largely because so many have already fled Mosul over the 11 years of turmoil and violence since the U.S. invasion in 2003.

A few large families of Turkmen who had taken shelter in local schools remained fearful, days after their flight from the insurgents, and refused to give their names to a visiting reporter.

“We’ll accept any way to solve this crisis,” one man said, sitting with his relatives on the floor of the kindergarten classroom where they now sleep.

Many said they were used to the hardship. Explosions, assassinations and kidnappings have rattled Mosul for years, and the violence worsened in the months before Sunni militants seized the city. But a sense of utter hopelessness also seemed to permeate the conversations of this latest influx of refugees.

“Iraq has ceased to be a state,” said Abdel Hady, the taxi driver at Garmava camp. Maybe with renewed American intervention, or some political wrangling in Baghdad, the country might hold together under one flag, he said. “But there will always be a war.”