WASHINGTON, Jan.25 (Xinhua) -- U.S. President Donald Trump on Wednesday signed an executive order to boost border security before his scheduled speech at the Department of Homeland Security.

"The first order is the border security and immigration enforcement improvements, it addresses long-overdue border security issues," White House Press Secretary Sean Spicer said during a press briefing. "It will be the first order in that to build a large physical barrier on the southern border."

Trump reiterated earlier Wednesday during an interview that the wall project will start as soon as possible and financed by Washington, but Mexico will "100 percent" reimburse the United States at "a later date."

The cost of the wall to block off the nearly 2000 mile (3200 km) long border has been estimated to be anything from eight billion U.S. dollars to 25 billion dollars. In some parts of the border, different versions of barriers already exist, but most of the border cross through inhospitable wilderness.

"The second order is enhancing the security of the interior of the United States," Spicer said.

"Federal agencies will be unapologetically enforce the law, no ifs, ands and buts," he said.

Spicer said the U.S. government will implement a host of measures to curb illegal immigrants entering the country.

The policy of "catch and release", under which illegal immigrants caught will be released quickly into Mexico, will be substituted with a longer deportation process, according to the new order.

Cities that take in illegal immigrants, also known as "sanctuary cities", will be banned from receiving federal grants, Spicer said.

The U.S. government will also withhold visas from countries that refuse to take back illegal immigrants.

Toughening up against illegal immigrants was a major part of Trump's campaign platform, and has attracted the support of large number of voters who fear their jobs are at risk with the influx of immigrants.

Since Trump has come into office, he has signed an executive order to withdraw the United States from the Trans-Pacific Partnership, and the Republican-controlled Congress has begun the process of repealing and replacing Obamacare.