US budget shutdown 'I won't compromise on the wall', says Trump

President Donald Trump has refused to compromise on his demand for a wall to be built on the Mexican border, the cause of a partial government shutdown.

He said he did not think he would have to reduce the $5.6bn in funds he was demanding and repeated a threat to declare a national emergency.

His new chief of staff Mick Mulvaney said earlier he expected the shutdown to "drag on a lot longer".

Democrats refuse to back the funding, calling the wall "immoral".

Talks to end the partial shutdown will resume later on Sunday, with the stalemate now in its third week, affecting some 800,000 federal workers.

The wall was one of Trump's key campaign pledges, and he had said Mexico would pay for it, something Mexico has never agreed to.

He was speaking as he left the White House for Camp David, where he will discuss policy issues with aides.

"We have to build the wall," he said. "It's about safety; it's about security for our country... We have no choice... it's a very important battle to win."

Trump said he thought the Democrats "want a deal", adding, "the shutdown could end tomorrow or go on a long time. It depends on the Democrats."

But he also repeated an earlier threat to declare a national emergency "depending on what happens in the next few days".

US law allows him to direct military construction projects under those circumstances but the money would have to come from defence department funds, and there would inevitably be legal challenges by the Democrats that would drag out the matter.

Trump also repeated the idea of building the wall out of "steel instead of concrete", presumably in the hope that Democrats could then claim it was not a wall.

The idea has not gained traction with the Democrats so far.

Mulvaney told NBC's Meet the Press on Sunday that he thought the controversy was "going to drag on a lot longer. I think that's... by intention". He did not elaborate.

The talks between the Democrats and Republicans on the shutdown resumed on Sunday afternoon.

Both sides believe their core support backs their stance, so leeway is limited.

An idea that has been floated, reportedly by Trump adviser Jared Kushner at the meeting on Saturday, was to give the Democrats new measures on young immigrants called "Dreamers", in exchange for the wall funding.

Dreamers are immigrants who illegally entered the US as children. Democrats want to ensure that they are shielded from deportation but have refused to link the issue with a deal on wall funding.

Some conservatives are also reportedly alarmed at any offer on the Dreamers.

Pelosi said after the failed talks on Saturday her party would introduce piecemeal bills aimed at reopening certain government agencies, starting with the Internal Revenue Service and the Treasury Department.

"This action is necessary so that the American people can receive their tax refunds on schedule," she said.

The bills may amount to little more than extra pressure on the president.

Partial shutdowns occur when Congress cannot agree on a budget by a certain deadline, or the president refuses to sign it.

The partial shutdown happened on 22 December and a quarter of the government has been closed since, leaving some 800,000 workers either furloughed - a kind of temporary leave of absence - or working without pay.

The Senate had actually reached bipartisan agreement on a budget, but Trump then refused to back it, demanding the funding for the wall.

The House then passed a bill including funding for the wall, when the Republicans still had a majority there, but they could not get the necessary 60 votes in the 100-seat Senate.

When the Democrats took control of the new Congress this week, they passed spending bills to reopen the government. The leader of the Republican-controlled Senate, Mitch McConnell, immediately called the move "a time-wasting act of political posturing".

In Friday's news conference, Trump told reporters he might consider asking his cabinet to decline a $10,000 pay rise that is due to take effect because a pay freeze has expired as an inadvertent result of the shutdown.