Image copyright Getty Images

US President Donald Trump is set to argue that an immigration "crisis" requires his long-promised wall along the Mexican border.

Mr Trump will make his case in his first speech from the Oval Office at 21:00EST (0200 GMT Wednesday), ahead of a trip to the border on Thursday.

A partial government shutdown has been in effect for 17 days after lawmakers failed to break a budget impasse.

The president is insisting that $5bn (£4bn) be included for the border wall.

White House press secretary Sarah Huckabee Sanders said in a tweet that Mr Trump would use his visit to the border on Thursday to "meet with those on the front lines of the national security and humanitarian crisis".

Ahead of the speech on Tuesday, Vice-President Mike Pence said Mr Trump's decision to address the nation "comes from this president's deep desire to do his job to protect the American people".

What will Trump say?

Donald Trump says there is a growing crisis along the US-Mexico border that demands immediate and drastic action.

The challenge for Mr Trump is that he has been calling for a wall - a bigger, longer one - since he announced his presidential bid in 2015. That has made it hard for his opponents to believe there is a new situation that requires the same solution, except smaller and shorter.

What's more, administration officials have been vague - perhaps intentionally - on the numbers supporting their claims. They say 4,000 "terrorists" have been caught entering the US, but it turns out most stops happen at airports. There have been only a handful apprehended at the Mexican border (fewer than at the border with Canada).

The White House has recently painted this as a humanitarian problem - and Central American families are, in fact, fleeing poverty and violence. But can he make the case that a wall is the humanitarian answer, and not just a way of making refugee suffering less visible to Americans?

The president must convince a sceptical public that his wall is worth the pain the shutdown is causing government workers and the nation at large.

He has eight minutes on Tuesday to do it.

Media playback is unsupported on your device Media caption Five questions about Trump's border wall

What has been the reaction?

The decision by the major US TV networks to set aside at least eight minutes of airtime for the speech has sparked controversy and fuelled debate on social media.

Critics pointed out that the networks aired President George W Bush's prime-time address on immigration in 2006, but did not air one by President Barack Obama in 2014.

Democratic House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer announced on Tuesday that they would deliver a joint rebuttal from the halls of Congress after Mr Trump has spoken.

Some reporters have called on the White House to provide the text of his remarks in advance to be fact-checked.

Skip Twitter post by @brianstelter TV exec texts: "He calls us fake news all the time, but needs access to airwaves… If we give him the time, he'll deliver a fact-free screed without rebuttal. And if we don't give him the time, he'll call every network partisan. So we are damned if we do and damned if we don't." — Brian Stelter (@brianstelter) January 7, 2019 Report

On Monday, living former US presidents disputed Mr Trump's claim that some of them had privately told him they regretted not building a border wall during their administrations.

"I have not discussed the border wall with President Trump, and do not support him on the issue," former President Jimmy Carter said in a statement issued by the Carter Center on Monday.

A spokesman for George W Bush said he had never discussed the border wall with Mr Trump. A representative for Bill Clinton denied it and said that the two men had not spoken since Mr Trump took office, according to CBS.

Mr Obama has not explicitly denied Mr Trump's claim, but has extensively campaigned against the president on the issue.

In Mr Pence's interview with NBC on Tuesday, he sought to clarify the president's original claim, saying "that was his impression from previous administrations, previous presidents".

"I know I've seen clips of previous presidents talking about the importance of border security, the importance of addressing the issue of illegal immigration," he added.

Image copyright Getty Images Image caption Mr Trump will visit the US-Mexico border on Thursday

The White House has been under fire amid a partial government shutdown, which began on 22 December and affects a quarter of the government.

Some 800,000 federal employees have been temporarily laid off - or are working without pay.

Nine departments have been affected, including Homeland Security, Justice, Housing, Agriculture, Commerce, Interior and the Treasury.

Will Trump declare a national emergency?

US media are reporting that Mr Trump will not declare a national emergency on Tuesday evening, in a bid to launch construction of a wall by circumventing Congress.

US law allows the president to direct military construction projects during war or national emergency, but that money would have to come from defence department funds allocated by Congress for other purposes, the BBC's Anthony Zurcher says.

Such a move would prompt members of Congress who are tasked with allocating tax money - including some Republicans - to push back, he says.

Curbing illegal immigration was one of the main campaign promises Mr Trump made when he ran for president in 2016.

During the campaign for the mid-term elections in November 2018, he repeatedly warned of a "caravan" of Central American migrants making its way towards the US-Mexico border.

In a reaction that many saw as a political ploy during a campaign season, he deployed 5,800 US troops to the border due to what he described as an "invasion".

Last month, Mr Trump suggested that the US military could be used to build the wall.