President Donald Trump will visit cities along the US-Mexico border on Thursday to renew calls for his desired wall.

He reportedly told TV anchors in private that the trip was just a photo opportunity and that "it's not going to change a damn thing."

The federal government has been partially shut down since December 22 over the disagreement between Trump and congressional Democrats on funding the wall.

The shutdown enters its 20th day on Thursday. If the shutdown is not resolved by Saturday, it will become the longest in US history.

President Donald Trump is heading to the US-Mexico border on Thursday to make a case for his proposed wall, despite reportedly dismissing the trip as a photo opportunity that was "not going to change a damn thing."

Trump plans to visit a border-patrol station in McAllen, Texas — one of the busiest parts of the border for illegal crossings. He will discuss immigration and border security and receive a security briefing on the border, the Associated Press reported.

It's not clear what the trip will achieve as far as changing lawmakers' minds. Trump, in private, reportedly dismissed the trip as a useless photo opportunity, saying, "It's not going to change a damn thing, but I'm still doing it."

Read more: 'How much more American blood must we shed': Trump delivers his first prime-time address from the Oval Office amid a government shutdown over funding for his border wall

Trump reportedly said the border trip was just a photo opportunity that was "not going to change a damn thing." Here he examines border-wall prototypes in San Diego in March. Associated Press/Evan Vucci

He also reportedly said the televised address he gave from the Oval Office on Tuesday was pointless.

The comments about both the Oval Office address and Thursday's trip were said to have come during an off-the-record lunch on Tuesday with television anchors, which was reported on by The New York Times.

On Thursday the federal government entered its 20th day of a partial shutdown over a disagreement between Trump and congressional Democrats over funding for the wall.

It is close to becoming the longest shutdown in US history — the record stands at 21 days, which took place in 1995-1996.

The president is said to have stormed out of a meeting over the government shutdown on Wednesday after Democratic leaders once against refused to fund a wall or similar steel barrier on the US-Mexico border.

Senate Minority Leader Chuck Schumer said Trump "sort of slammed the table" and walked out of the room when House Speaker Nancy Pelosi said no to funding for a wall or steel barrier.

Trump gave a prime-time televised address over the border on Tuesday night. Associated Press/Carolyn Kaster

Trump has also repeatedly floated the idea of declaring a national emergency to start construction on a wall on the border in an effort to circumvent Congress. Democrats have said he has no legal basis to do this, and Republicans are uneasy about the issue.

He has also reportedly instructed the White House Office of Management and Budget to explore whether he can use the Department of Defense's budget to fund the wall without requiring Congress' permission.