A Homeland Security official told The Associated Press that about 2,000 mattresses have been ordered to the makeshift holding center - a warehouse that has not been used to house people in years.
Gov. Jan Brewer's spokesman, Andrew Wilder, said Friday that conditions at the center are so dire that federal officials have asked the state to immediately ship medical supplies to the center in Nogales.
The U.S. Department of Homeland Security started flying immigrants to Arizona from the Rio Grande Valley in Texas last month after the number of immigrants - including more than 48,000 children traveling on their own - overwhelmed the Border Patrol there.
Immigrant families were flown from Texas, released in Arizona, and told to report to an Immigration and Customs Enforcement (ICE) office near where they were traveling within 15 days. ICE has said the immigrants were mostly families from Central America fleeing extreme poverty and violence.
The official, who spoke on condition of anonymity because there was no authorization to discuss the matter publicly, said the holding center opened for unaccompanied children because the U.S. Department of Health and Human Services had nowhere to turn.
"They became so overwhelmed and haven't kept up with planning," the official said.
At the holding center, vendors are being contracted to provide nutritional meals, according to U.S. Customs and Border Protection officials. The Federal Emergency Management Agency, meanwhile, will provide counseling services and recreational activities.
The Homeland Security official said the number of children at the warehouse was expected to double to around 1,400. The warehouse has a capacity of about 1,500.
The warehouse began housing children flown from South Texas last Saturday. About 400 were scheduled to arrive Friday but, because of mechanical issues with the planes, only about 60 came, the Homeland Security official said. Saturday's flights were canceled, also because of mechanical problems. There are flights scheduled through mid-June.
Federal authorities plan to use the Nogales facility as a way station, where the children will be vaccinated and checked medically. They will then be sent to facilities being set up in Ventura, California; San Antonio, Texas; and Fort Sill, Oklahoma.
The Homeland Security official said that the children would be moved out of the Nogales site as soon as Health and Human Services finds places for them.
But the official said: "As quickly as we move them out, we get more. We believe this is just a start."
The children being held in Nogales are 17 or younger. The official estimated three of every four were at least 16.
Wilder said reports from consulates that the Department of Homeland Security was stopping the program to fly migrant families to Arizona and then bus them to Phoenix were incorrect. Instead, the program that has shipped unknown thousands of adult migrants and their children to Arizona since last month shows no sign of stopping, he said.
"The adults, the adults with children, families - that continues unfettered and we have no idea where they are going," Wilder said.
In a statement Friday, Homeland Security officials said "appropriate custody determinations will be made on a case by case basis" for migrants apprehended in South Texas.
Brewer sent an angry letter to President Barack Obama on Monday demanding that the program of dropping off families at bus stations in Phoenix stop immediately. She called the program dangerous and unconscionable, asked for details and demanded to know why state authorities weren't consulted or even informed.
The governor said she hadn't received a response to her letter by Friday.
"I have reached out to Federal Homeland Security Director Jeh Johnson for answers. Meanwhile, I reiterate my call on President Obama to secure our southern border and terminate this operation immediately," Brewer said in a statement.
Brewer's staff spent Friday in a series of calls with officials from FEMA, Immigration and Customs Enforcement and Homeland Security.
Wilder said FEMA's Region 9 administrator was being sent to the holding center in Nogales on Saturday to oversee efforts to deal with the hundreds of arriving children.
The federal emergency supplies are held in Arizona warehouses, and Wilder said the state is working to send them to the holding center.
On Friday night, the U.S. Department of Justice announced that young lawyers and paralegals are being sought for the community service program AmeriCorps to provide legal assistance in immigration proceedings to children who come to the U.S. illegally. Officials say about 100 lawyers and paralegals will be enrolled as members of AmeriCorps in a new division called "justice AmeriCorps."
Immigration officials can immediately return Mexican immigrants to the border, but they are much more hard-pressed to deal with Central American migrants who illegally cross into the U.S. In recent months, waves of migrants from nations south of Mexico have arrived in Texas.
The Homeland Security official said that legally, only their parents or guardians can take custody if the government makes the children eligible for release.
Officials in Central America and Mexico have noticed a recent increase in women and children crossing the border. Father Heyman Vazquez, the director of a migrant shelter in Huixtla in the southern Mexico state of Chiapas, said he and others advise children that it's too dangerous.
Yet Vazquez is seeing more and more youths heading north.
"I remember a little boy of 9 years old and I asked if he was going to go meet someone and he told me 'No, I'm just going hand myself over because I hear they help kids,' " Vazquez said.
The perception that some immigrants could be getting a free pass into the U.S. could lead to more attempts to cross the border.
Illegal immigration increased heavily under a "catch-and-release" strategy during the George W. Bush administration. Under that policy the government issued notices to appear in immigration court to migrants from countries other than Mexico until Bush stopped the practice.
Federal officials then established a 210-mile stretch of the Texas-Mexico border as a zero-tolerance zone for illegal immigration. Instead of merely getting sent back home, migrants were arrested, prosecuted and sometimes sentenced to prison before being formally kicked out of the country.
By August 2006, border agents in the Del Rio, Texas, sector said daily arrests had dropped from 500 to fewer than 100.