President Donald Trump stood firm Sunday on his demand for billions of dollars to fund a border wall with Mexico, which has forced a shutdown of the US government now entering its third week.

"We have to build the wall," Trump told reporters as he left the White House for the Camp David presidential retreat, while conceding that the barrier could be "steel instead of concrete".

"It's about safety, it's about security for our country," he said. "We have no choice."

An impasse with lawmakers over funding for the border wall has partially shut down the federal government since December 22.

Talks aimed at ending the shutdown were to continue Sunday, after discussions a day earlier between Vice President Mike Pence and representatives of Chuck Schumer and Nancy Pelosi, the top two Democrats in Congress, made little headway.

Democrats, who now control the House of Representatives, seem in no mood to make concessions to the president on a border wall Pelosi has described as an "immorality".

"This shutdown could end tomorrow and it could also go on for a long time," Trump said. "It really depends on the Democrats." Both Democrats and Republicans have attempted to pin the blame for the shutdown on the other side.

Building a wall along the 2,000-mile (3,200-kilometre) US-Mexico border was a central plank in the 2016 election campaign of Trump, who has sought to equate immigrants with crime, drugs and gangs.

The president has described the situation at the border as a "national emergency," and has said he is looking into ways to get a wall built without congressional approval.

(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)