US President Donald Trump warned on Wednesday the US federal government may not reopen any time soon, as he stood firm on his demand for billions of dollars in funding for a border wall with Mexico.

Addressing a cabinet meeting on the 12th day of the shutdown, Trump warned it "could be a long time" before the impasse is resolved.

"It's too important a subject to walk away from," said the president, who was to meet later with Democratic and Republican lawmakers.

"The 5.6 billion is such a small amount and we are talking about national security," Trump added, saying: "I think the people of this country think I am right."