Donald Trump has blamed the Democrats for the recent deaths of two immigrant children in US custody as the second week of a government shutdown begins.
The US president said the deaths of Felipe Gomez Alonso and Jakelin Caal, aged eight and seven, were "strictly the fault of the Democrats and their pathetic immigration policies that allow people to make the long trek thinking they can enter our country immediately".
Both children were from Guatemala but died in separate circumstances after crossing the border illegally and being taken into the custody of US Border Patrol.
In his first comments on their deaths, Mr Trump said they were "very sick" before they were taken into custody. However, immigration officials have said both children passed initial health checks – with Felipe's mother telling the AP news agency that her son was healthy before he began his journey to the US.
Felipe collapsed with a fever after arriving with his father from one of Guatemala's indigenous communities.
His death on Christmas Day came as Jakelin was being buried back in Guatemala.
She died of dehydration and shock after being arrested about 90 miles north of the border in New Mexico.
They were among 139,817 people arrested near the US southwest border in the past two months, almost twice the number seen during the same period last year – 14,000 of those unaccompanied children.
Washington is currently in the throes of a partial government shutdown, triggered over Mr Trump's demands for billions of dollars in funding to build a wall along the US-Mexico border.
On Twitter, he argued that migrants "wouldn't even try" to make the crossing if the wall was in place, and also wrote: "I am in the White House waiting for the Democrats to come on over and make a deal."
There have been no signs of substantive negotiations between Mr Trump, his Republican party and the Democrats – even though the Republicans only hold a monopoly on power in Washington for just five more days.
One Democratic representative, Dwight Evans of Pennsylvania, warned: "[Mr Trump is] reaching new lows with these ridiculous tweets. His administration is the cause of pain and suffering that is taking place at the border. Nothing that he says will alter this truth."
Others called his tweets "disturbing" and pointed out that US immigration policies have been in place for years without children dying in government custody.
Homeland Security secretary Kirstjen Nielsen travelled to Arizona on Saturday to meet medical staff at the border.
She said in a statement that "the system is clearly overwhelmed and we must work together to address this humanitarian crisis".
Mr Trump is demanding $5bn (£3.9bn) in funding for the wall, and has described it as an urgent priority.
But Democrats have only agreed to offer $1.3bn (£1.02bn) in funding for security, which would not include cash to build the wall.
The standoff has resulted in the third government shutdown of the year, with more than 800,000 government employees sent home or made to work without pay.
Many of them have been on holiday over the Christmas period, but Mr Trump is likely to come under renewed pressure to reach a compromise if the shutdown continues beyond the New Year.
Several organisations have started to run out of money and have implemented their shutdown procedures – with the Environmental Protection Agency furloughing most of its 14,000 employees as of Friday night. Only workers deemed essential for preventing public health threats will stay on the job.
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Elsewhere, the Smithsonian Institution has warned its museums, art galleries and zoo will begin to close in the middle of the week if the shutdown drags on.