By Doina Chiacu and Richard Cowan

WASHINGTON (Reuters) - Border security will probably suffer if Congress fails to act on President Barack Obama's request for extra money to cope with the explosion of child migrants into the U.S. Southwest, lawmakers and congressional aides warned on Tuesday.

Those concerns came as Republicans in the House of Representatives considered a severely scaled-back version of the $3.7 billion in emergency funding Obama has requested to deal with the border crisis.

The Immigration and Customs Enforcement agency is on pace to run out of money in mid-August, and U.S. Customs and Border Protection in mid-September, Homeland Security Secretary Jeh Johnson has said.

If extra funding is not provided, the government will have to transfer money from other agencies to deal with crisis at the border, where 57,000 children traveling alone have tried to enter the United States from Central America this year.

The transfer of funds is known as "reprogramming" and can be carried out by the Obama administration without legislation.

Senate Homeland Security Chairman Tom Carper, a Delaware Democrat, was "deeply concerned" that forcing the Department of Homeland Security (DHS) to move money around will affect other critical operations, according to a Democratic committee aide.

Border security will be hurt because DHS probably would need to reduce aerial support for the Border Patrol and put off buying technology needed at border points, according to Democratic and Republican congressional aides.

"We also understand that it would hinder some of the very functions critics of the supplemental (funding) seem to support: expedited detentions and deportations of undocumented migrants," the Democratic aide said on condition of anonymity.

House Homeland Security Committee Chairman Michael McCaul, a Texas Republican, said he understood that reprogramming would take money from the Coast Guard and cybersecurity, "and so I am very disturbed at the administration's reprogramming approach."

McCaul said he was meeting with DHS chief Johnson on the issue later on Tuesday. He said House Republicans want him to take money out of Federal Emergency Management Agency operations instead of border security.

The House bill would provide $659 million through the end of the fiscal year, Sept. 30, which is also sharply lower than the $2.7 billion the Senate wants.

But a bigger sticking point will be House Republicans' proposed changes to a 2008 human trafficking law to allow quicker deportation of children from Honduras, Guatemala and El Salvador, something the Democratic-controlled Senate is unlikely to agree to.

With some conservative House Republicans expressing opposition to spending the additional money and wanting even tougher immigration policy changes attached to the bill, there were questions over whether the measure can pass the House this week.

(Additional reporting by David Lawder and Emily Stephenson; Editing by Jonathan Oatis)