President Donald Trump in a rare Oval Office address to the nation on Wednesday demanded funding for his long-promised US-Mexico border wall to halt "a growing humanitarian and security crisis" due to "uncontrolled and illegal" migration, but refrained from declaring a national emergency over the issue.

Trump's remarks came amid a partial government shutdown that is days away from becoming the longest in US history.

The president sought to pressure Democrats to agree to his request for USD 5.7 billion for the wall as a condition of ending the government shutdown.

He also tried to ramp up support among Republicans who are getting nervous about government workers and others who are feeling the pain of the shutdown.

"This is a humanitarian crisis. A crisis of the heart, and a crisis of the soul," Trump said in his maiden prime-time address from his Oval Office.

Noting that America proudly welcomes millions of lawful immigrants who enrich the American society and contribute to the nation, Trump said all Americans are hurt by uncontrolled, illegal migration.

"It strains public resources and drives down jobs and wages. Among those hardest hits are African-Americans and Hispanic Americans," he said in an eight-minute address as he sought to build a physical barrier along the southern Mexico border.

Funding for a border wall has been the main sticking point in negotiations with Democrats to reopen parts of the federal government that have been closed for 18 days.

A wall along the US-Mexico border was one of Trump's signature campaign promises. Trump wants USD 5.7 billion to build the wall, even though he said repeatedly during his campaign that he would make Mexico pay for the structure. Democrats are refusing to allocate the money, arguing that a wall would be costly and ineffective.

The standoff triggered a partial government shutdown on December 22, shuttering nine federal departments and several smaller agencies and forcing some 800,000 workers to go on unpaid leave or work without pay. The shutdown will enter its 19th day Wednesday, making it the second-longest in history.

Trump in the past few days had threatened to declare a national emergency over the issues that would have allowed him to start building the wall with military funds without congressional authorisation.

But in Wednesday's address he stopped short of declaring a national emergency.

Making a strong pitch for the wall, Trump said the federal government remained shut because of Democrats.

The president also said that 90 per cent of heroin sold in the US came from Mexico.

"Last month 20,000 migrant children were illegally brought into the United States, a dramatic increase. These children are used as human pawns by vicious coyotes and ruthless gangs. One in three women are sexually assaulted on the dangerous trek up through Mexico," Trump said.

"Women or children are the biggest victims by far of the broken system. This is the tragic reality of illegal immigration on our southern border. This is the cycle of human suffering that I am determined to end," he said.

"As part of an overall approach to border security law enforcement professionals have requested $5.7 billion for a physical barrier. At the request of Democrats, it will be a steel barrier rather than a concrete wall. This barrier is absolutely critical to border security. It's also what are professionals at the border want and need,"Trump said.

Trump said illegal immigrants and drugs flowing across the southern border posed a serious threat to American safety.

"The border wall would very quickly pay for itself. The cost of illegal drugs exceeds USD500 billion a year, vastly more than the USD5.7 billion we have requested from Congress,” Trump said.

The opposition Democrats immediately dismissed Trump's assertion on border security and accused the president of holding the nation hostage to a partial government shutdown for more than a fortnight now.

"Much of what we have heard from President Trump throughout this senseless shutdown has been full of misinformation and even malice. The President has chosen fear," House Speaker Nancy Pelosi said in the Democratic address to the nation immediately after Trump's prime time address.

"President Trump has chosen to hold hostage critical services for the health, safety and well-being of the American people and withhold the paychecks of 800,000 innocent workers across the nation – many of them veterans," she alleged.

Senate Minority Leader Senator Chuck Schumer said the American democracy doesn't work the way Trump wants.

"We don't govern by temper tantrum.? No president should pound the table and demand he gets his way or else the government shuts down, hurting millions of Americans who are treated as leverage," he said.

PTI