US Secretary of Homeland Security Jeh Johnson visited a temporary facility for detained migrants in Artesia, New Mexico, where he says the migrants will remain until they are deported back to their home countries.

"Our message is clear. If you come to this country illegally into the Rio Grande Valley sector, we intend to send you back consistent with our laws and we are building additional capability to do that quicker and safely," Johnson told reporters outside the facility.

The US is facing an immigration crisis with thousands of Central American children who migrated illegally into the country at the southwest US border and unaccompanied by adults.

The Obama administration put forward this week a request for emergency funding of $3.7 billion to address the issue.

In a letter to congressional leaders last week, President Obama proposed giving the Department of Homeland Security additional authority to process the return and removal of unaccompanied children from Guatemala, Honduras and El Salvador.

After visiting the facility, Johnson said the department is trying to expedite the removal process to deal with the crisis quickly.

"I got a report from the personnel who are working here about what they are hearing from the migrants who are here, and I think it's fair to say that a good number of them were surprised that they were being detained," Johnson said. "I think they expected to be apprehended and simply let go into the interior, and they are surprised that they are being detained and sent back so quickly."