WASHINGTON (Reuters) - The Obama administration on Monday announced it is designating a third U.S. military base for emergency housing of children immigrating illegally into the United States without parents or relatives, as the cost of caring for these minors escalated.

Senior administration officials, who asked not to be identified, told reporters that an Army base at Fort Sill, Oklahoma, will initially hold 600 "unaccompanied minors" and eventually will be able to accommodate up to 1,200.

In recent weeks, the Obama administration has opened similar emergency shelters at Lackland Air Force Base in San Antonio, Texas, and Naval Base Ventura County in Southern California.

The moves come amid a tidal wave of children trying to slip into the United States, largely from Honduras, El Salvador and Guatemala, often to join a parent already here.

Reuters previously reported that the administration was seeking about $2 billion for the Department of Health and Human Services to handle the influx in fiscal 2015, which begins on Oct. 1, more than double the $868 million appropriated this year.

HHS takes custody of the children shortly after they are detained at the border by federal law enforcement agents.

On Monday, administration officials said they would be asking Congress for an additional $560 million to help the Department of Homeland Security cope with the illegal border crossings.

One week ago, the White House director of domestic policy, Cecilia Munoz, attributed the rapid run-up in illegal immigration by unaccompanied minors to growing violence -- often drug related or due to domestic abuse -- in the three Central American countries.

That violence, she said, was encouraging children, including an unusually high number of girls and children under the age of 13, to leave home unaccompanied by parents or relatives.

Administration officials have been countering suggestions by political opponents that the influx could be related to either Obama administration policies or legislation pending in the U.S. Congress to revamp immigration law.

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One of the officials, noting high murder rates in Honduras, Guatemala and El Salvador, said, "If they (unaccompanied minors) are coming solely because of immigration policies, you would see large numbers from other countries (in Central America), including Mexico as well."

The officials did not provide figures on how many children the U.S. government was holding in detention and putting through deportation proceedings.

Besides opening new shelter facilities, the administration has announced that it is working with Mexican and Central American governments to try to discourage children from making the dangerous journey to the United States and to try to further secure borders.

(Reporting by Richard Cowan; Editing by Leslie Adler)