U.S. President Donald Trump on Friday declared a national emergency to secure funds for a wall on the border with Mexico without congressional approval, a controversial move that appears certain to draw a court challenge from Democrats.

With the declaration, Trump is seeking to divert about $6.5 billion in funds from military and other sources through executive action, adding to a smaller amount allocated via a border security bill that he signed to avert another government shutdown.

"A national emergency exists at the southern border of the United States," Trump declared, citing the border as "a major entry point for criminals, gang members and illicit narcotics."

"The current situation at the southern border presents a border security and humanitarian crisis that threatens core national security interests and constitutes a national emergency," he said in a statement.

Referring to a signature promise of Trump's 2016 presidential campaign and with his 2020 re-election bid in sight, the White House said the U.S. leader "is following through on his promise to secure the border with legislation and executive action." Trump, however, repeatedly pledged on the campaign trail that Mexico would pay for the wall.

The legislation includes $1.375 billion for 88 kilometers of fencing and other physical barriers at the southern border -- far short of the $5.7 billion Trump had demanded for more than 320 kilometers of steel and concrete wall.

Democrats, who control the House of Representatives, condemned the Republican president's move, saying it clearly violates Congress' exclusive control over the national purse.

"The president's unlawful declaration over a crisis that does not exist does great violence to our Constitution and makes America less safe, stealing from urgently needed defense funds for the security of our military and our nation," House Speaker Nancy Pelosi and Senate Democratic leader Chuck Schumer said in a joint statement.

"The Congress will defend our constitutional authorities in the Congress, in the courts, and in the public, using every remedy available," the statement said.

Trump acknowledged his action would face a lengthy court battle, but expressed confidence that he will eventually prevail in the Supreme Court.

"We will possibly get a bad ruling, and then we'll get another bad ruling," he told reporters at the White House. "And then we'll end up in the Supreme Court, and hopefully we'll get a fair shake. And we'll win in the Supreme Court."

Referring to Democratic leaders, Trump said, "They say walls don't work. Walls work, 100 percent" in blocking what he called "an invasion of drugs, invasion of gangs, invasion of people."

"If you're going to have drugs pouring across the border, if you're going to have human traffickers pouring across the border in areas where we have no protection, in areas where we don't have a barrier, then very hard to make America great again," he said.

Trump's demand for $5.7 billion in funding for the wall led to a 35-day partial government shutdown through Jan. 25, the longest in U.S. history.