U.S. President Donald Trump on Sunday floated an idea of declaring a national emergency to build his long-promised border wall without congressional approval, as talks on it and ending a partial government shutdown continued.

"I may decide a national emergency depending on what happens over the next few days," Trump told reporters at the White House before leaving for Camp David, a presidential retreat in the state of Maryland, for meetings with senior White House staff on border security and other issues.

The president said that a wall or a barrier must be built along the U.S. southern border with Mexico, while signaling a willingness to accept a steel barrier instead of a concrete wall by saying that "steel instead of concrete is fine."

He has insisted that the wall is essential to addressing illegal immigration and drug trafficking, while Democrats have slapped the proposal as an "inefficient, unnecessary and costly" solution to strengthening border security.

The Sunday remarks came a day after negotiations between senior Trump administration officials and congressional Democratic staffer on the border wall funding and a budget standoff yielded what Trump described as "not much headway," as each side accused the other of giving no ground.

"This shutdown could end tomorrow or it also could go on for a long time," Trump said Sunday.

Mick Mulvaney, the acting White House chief of staff, told CNN's "State of the Union" program on Sunday that the negotiations held Saturday was "disappointing."

Trump is demanding over 5 billion U.S. dollars in border security to deliver his signature campaign promise to build the border wall, whose construction and funding have been strongly rejected by Democrats.

"We're asking for $5.6 billion. They're offering us zero," Mulvaney said.

White House press secretary Sarah Sanders, appearing on "Fox News Sunday", said the Trump administration is looking at "every option available" to get the needed money, including bypassing Congress if necessary by declaring a national emergency to build the border wall.

Senate Minority Whip Dick Durbin on Sunday warned of legal challenges if Trump tries circumventing Congress.

"I don't know what he's basing this on, but he's faced so many lawsuits when he ignores the law and ignores tradition and precedent and just goes forward without any concern," the Democrat said on CBS "Face the Nation."

Sunday is the 16th day of the ongoing partial government shutdown, which has affected nine cabinet-level departments and dozens of agencies as well as the jobs and paychecks of some 800,000 federal employees.

House Speaker Nancy Pelosi said in a statement on Saturday that Democrats in the House, who now hold a majority, will take action next week to reopen the government by approving individual funding bills for agencies affected by the shutdown.

The fate of those Democratic bills is unclear. Senate Majority Leader Mitch McConnell has said the Republican-controlled upper chamber won't consider any bills without the president's support.