US President Donald Trump has said that for now, he would not declare a national emergency as a way of securing funds for a border wall and ending a partial government shutdown, adding that he would rather see the US Congress act.

"We want Congress to do its job," Trump said on Friday during a White House event on border security.

"What we're not looking to do right now is national emergency," the Republican president said.

Key parts of the US government shut down on December 22 after funding expired as Trump and congressional Democrats quarrelled over his demand for $5.7bn in funding to build a wall along the US-Mexico border.

Trump repeated his view that he had the right to declare a national emergency to divert funds appropriated for other purposes but said, "I'm not going to do it so fast."

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He also reiterated that he was open to considering a major immigration reform but that he would only do so after he obtained funding for the wall.

Democrats have refused to give in to Trump's proposed wall, and the shutdown - now in its 21st day - is expected to become the longest of its type in US history over the weekend.

Democrats view the wall as ineffective, expensive and immoral. Instead, they have agreed to provide more than $1.3bn in federal funding for broader border security measures that do not include a wall.

'Fight about border security'

Pressure is mounting on politicians to find a way to reopen the government. Some 800,000 federal workers are affected with about half required to work without pay and the other half furloughed. Many received pay stubs with nothing but zeros on Friday, deepening anxieties about mortgage payments and unpaid bills.

Congress has passed legislation that would ensure all federal employees are paid retroactively after the shutdown ends. The bill awaits the president's signature.

On Thursday, federal workers, several Democrats and unions gathered in Washington for a rally against the shutdown.

While addressing the crowd, US Senator Mark Warner, a Democrat from Virginia, chided Trump.

"You want to fight about border security? We'll have that discussion," he said. "But do not hold 800,000 people's lives as political pawns."

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On Wednesday, Trump reportedly stormed out of a meeting with leading Democrats Nancy Pelosi and Chuck Schumer after they refused to approved funds for the wall in exchange for ending the shutdown.

When he left for McAllen, Texas, on Thursday morning, there were no further shutdown negotiations scheduled.

The Democrat-controlled House has passed several pieces of legislation that would open specific government departments, but Republican leaders in the Senate have refused to bring any bill that Trump won't sign to a vote.