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President Donald Trump's Department of Homeland Security has requested $18 billion in funding to complete a wall on a portion of the U.S. southern border in an effort to stop illegal immigration, one congressional source and one U.S. official told NBC News on Friday.

The Wall Street Journal was the first to report the plan, which would include 316 miles of new fencing and 407 miles of reinforcing existing fence over the next decade.

Prototypes for U.S. President Donald Trump's border wall with Mexico are shown near completion in this picture taken from the Mexican side of the border, in Tijuana, Mexico, Oct. 23, 2017. Jorge Duenes / Reuters file

Construction of the U.S.-Mexico border wall was a key campaign promise of Trump's. At the time, he estimated the construction would cost $10 billion and cover the entire 2,000-mile border. Over 350 miles of the southern border are already blocked by physical barrier.

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So far, the Trump administration has struggled to secure funding to cover just 72 miles of priority areas along the border, which have been identified near San Diego and the Rio Grande Valley.

Trump has pressured Democrats in Congress to approve the funding in exchange for allowing undocumented immigrants who were brought to the United States as children, and who had been protected by the Obama-era program known as DACA, to remain.

Sen. Dick Durbin, D-Ill., the minority whip, called the $18 billion request "outrageous."

"President Trump has said he may need a good government shutdown to get his wall. With this demand, he seems to be heading in that direction," Durbin said in a statement. He said that the administration was undercutting bipartisan efforts to help those protected by DACA with the $18 billion request and other immigration demands.

The congressional source told NBC News that the $18 billion request is not likely to be dealt with in upcoming funding discussions.