Donald Trump claimed Democrats "just want illegal immigrants to pour into our nation unchecked"
Democrats and Republicans in the US appeared locked in a 'blame game' as they entered the second day of a government shutdown.
The two parties have been blaming each other for the impasse that has resulted in senators being unable to pass a spending bill to fund government services, with President Trump insisting he will not negotiate with Democrats until they 'stop playing games'.
A government shutdown sees many federal employees sent home (or 'furloughed') while others - including members of the military - may not be paid.
An estimated 850,000 workers, out of a total 3.5 million, could be told to stay home until politicians manage to reach agreement.
The two major US parties have been locked in a stalemate over a number of issues, most prominently Democrat-led efforts to secure new protections for young undocumented immigrants known as 'Dreamers'.
The Senate - which the Republicans narrowly control with 51 seats - reconvened on Saturday, and senators are due to meet again on Sunday amid the continuing efforts to find a bipartisan solution to the issue.
While a majority of Democratic senators opposed the short-term spending bill put forward on Friday, a small number of Republicans also voted against the measure - instead calling for a long-term deal rather than 'stopgap' solutions.
Over the weekend, the blame game continued between party leaders - with Donald Trump repeatedly pointing the finger at Democrats.
The president spent the first anniversary of his inauguration in the White House after being forced to cancel a planned celebration at his Mar-a-Lago resort in Florida due to the political turmoil in Washington DC.
On Saturday, he claimed: "Democrats are holding our Military hostage over their desire to have unchecked illegal immigration. Can’t let that happen!"
In a tweet early on Sunday, he threatened Democrats with the 'nuclear option' - a rarely-used and controversial procedure to alter US Senate rules to require a simple majority rather than the three-fifths (60 senators) vote typically needed:
Great to see how hard Republicans are fighting for our Military and Safety at the Border. The Dems just want illegal immigrants to pour into our nation unchecked. If stalemate continues, Republicans should go to 51% (Nuclear Option) and vote on real, long term budget, no C.R.’s!— Donald J. Trump (@realDonaldTrump) January 21, 2018
On Saturday, White House press secretary Sarah Huckabee Sanders said: "The President will not negotiate on immigration reform until Democrats stop playing games and re-open the government."
Senate minority leader Chuck Schumer, the leading Democrat in the upper house, had met with President Trump to try and reach a deal on Friday - but the efforts appeared to have stalled.
On Saturday, Mr Schumer suggested: "Negotiating with President Trump is like negotiating with jello. That's why this compromise will be called a Trump Shutdown.
“[Negotiating is] next to impossible. As soon as you take one step forward, the hard-right forces the president three steps back.”
Majority leader Mitch McConnell, meanwhile, argued: "Shutdowns have consequences. The Democratic Leader may be playing for political points.
"But the rest of us understand that the readiness of our armed forces, health coverage for poor children, and survivor benefits for the families of fallen service members are the farthest thing from a game. Playing with all of those lives over the issue of illegal immigration."
In contrast, House Democratic leader Nancy Pelosi accused Republicans of being "so incompetent and negligent that they couldn't get it together to keep the government open".
The current impasse marks the first government shutdown since 2013, when a dispute over healthcare legislation led to a 16-day shutdown.