US President Donald Trump made a case for a wall on the border with Mexico in a televised address on Tuesday, saying the situation is a "humanitarian crisis - a crisis of the heart and a crisis of the soul".

The address came before a rebuttal by top Democrats, Nancy Pelosi and Chuck Schumer, who said the president "has appealed to fear, not facts".

The speeches came on the 18th day of a partial government shutdown, centred on Trump's demand for more than $5bn in wall funding, a request Democrats vehemently oppose.

"This is a choice between right and wrong; between justice and injustice," Trump said in his first address from the Oval Office. Urging Democrats to return to the White House to meet for talks, Trump said it was "immoral" for "politicians to do nothing".

In the Democratic rebuttal, Pelosi said Trump "must stop holding the American people hostage, must stop manufacturing a crisis and must re-open the government".

Trump cited cases of Americans "savagely murdered in cold blood" by undocumented immigrants.

Rubintino Sanchez, a migrant from Mexico, also challenged Trump's assertions that most migrants and refugees coming into the country are law-breakers.

"We aren't criminals. The three or four examples he mentioned, but it doesn't mean much. He doesn't explain whether the drugs these Hispanic people consumed turned them into criminals or not."

Trump also said the wall was needed to stop the flow of drugs that kill many Americans each year. Critics and rights groups, however, point out that a large percentage of illegal drugs, including the majority of heroin, enter the US via official ports of entry and a wall would likely have little to no effect on the number of drugs passing through the border.

Despite several attempts at talks, Trump and his fellow Republicans and Democrats have so far failed to find to a deal on the funding, and Tuesday's speeches deepened the feeling that politicians are still far from finding a solution.

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The partial shutdown, which began on December 22, affects some 800,000 federal workers in nine departments and several agencies. Employees have either been furloughed or are required to work without pay.

The shutdown has also strained the immigration system, worsening backlogs in courts and complicating hiring for employers.

Growing proportion of Americans blame Trump: poll

Trump maintains that the border wall is needed to stem irregular immigration and the flow of illegal drugs across the border, branding the issue as a "humanitarian crisis" and security concern. But critics argue that the security risks are overblown and the Trump administration, who has implemented a "zero-tolerance" policy towards immigration, is at least partly to blame for the humanitarian situation.

A Reuters/Ipsos poll, released on Tuesday, found that a growing proportion of Americans blame Trump for the shutdown, though Republicans mostly support his refusal to approve a budget without taxpayer dollars for a wall on the US-Mexico border.

The national opinion poll, which ran from January 1 to January 7, found that 51 percent of adults believe Trump "deserves most of the blame" for the shutdown. That is up four percentage points from a similar poll that ran from December 21 to 25.

Another 32 percent blame congressional Democrats for the shutdown and seven percent blame congressional Republicans, according to the poll. Those percentages are mostly unchanged from the previous poll.

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Trump has warned the shutdown could last a "long time", and said last week he could declare a national emergency to bypass Congress to build the wall. But he did not mention a national emergency during Tuesday's address. If he followed through with the threat, it would likely be challenged in the courts. Trump will visit the border on Thursday.

'Stop holding federal workers hostage'

Democrats blame Trump for the shutdown. They view the border wall as expensive and ineffective and instead support other security measures.

Hours after taking control of the House on January 3, Democrats passed a two-bill spending package aimed at immediately re-opening the government. This includes $1.3bn for border fencing and $300m for other border security items such as technology and cameras.

Senate Majority Leader Mitch McConnell, a Republican, has so far this year refused to bring any legislation Trump won't sign to a vote.

McConnell faces increasing pressure from within his party, especially from vulnerable Republicans up for re-election in 2020, as several conservative senators urged action to reopen the government, according to US media.

Prior to the address, several Democratic members of Congress took to Twitter to slam Trump for positioning the shutdown as a "humanitarian crisis" issue.

New House Democrat Ilhan Omar tweeted, "A wall won't solve the 'humanitarian crisis'. A wall won't pay the bills of 800,000 fed employees. A wall won't fix our broken immigration system. Show leadership and pass the bill Dems sent to reopen the government. Stop holding gov employees hostage for a xenophobic wall!"

A wall wont solve the "humanitarian crisis".



A wall won't pay the bills of 800,000 fed employees.



A wall won't fix our broken immigration system.



Show leadership and pass the bill Dems sent to reopen the government.



Stop holding gov employees hostage for a xenophobic wall! https://t.co/hG0FooFCv8 — Ilhan Omar (@IlhanMN) January 8, 2019

Her colleague Pramila Jayapal, who has been an outspoken critic of Trump's zero-tolerance policy towards immigration, blamed Trump for creating any crisis by "instituting #FamilySeparation, 'turn-backs' & shutting down asylum seekers."

She added, "Reverse those policies...crisis fixed! Meanwhile, 800K workers suffer b/c of 'vanity wall.' #TrumpShutdown."

Good grief. Trump now says there's a "humanitarian" crisis on border. Ummm...he created that by instituting #FamilySeparation, "turnbacks" & shutting down asylum seekers. Reverse those policies...crisis fixed! Meanwhile, 800K workers suffer b/c of "vanity wall." #TrumpShutdown — Rep. Pramila Jayapal (@RepJayapal) January 8, 2019

The US government has shut down fully or partially 21 times since 1976. The ongoing funding gap is tied with the second-longest on record.