Republicans and Democrats claimed that progress had been made in a weekend of talks on the issue, but they cancelled a scheduled late-night Sunday vote on a stopgap funding measure. Now the vote is expected to take place on Monday afternoon (1700 GMT).
US President Donald Trump has accused opposition Democrats of taking Americans hostage by their demands, which, according to him, triggered the government shutdown.
He accused them of placing immigrants ahead of American citizens and shutting down the government in the service of their base.
“Democrats have shut down our government in the interests of their far left base. They don’t want to do it but are powerless!” he tweeted on Monday, referring to the Democratic leadership in Congress.
The government shutdown, first in five years, went into effect shortly after Friday midnight, when Senate Republicans and Democrats failed to reach an agreement on the stopgap funding measure to continue government services.
The shutdown followed Congress’ failure to reach a deal on the status of “Dreamers” – hundreds of thousands of undocumented immigrants who entered the US as children.
The deadlock had already cast a huge shadow over the first anniversary of Trump's inauguration as president.
The rival lawmakers traded bitter recriminations on a special weekend sessions of Congress on Sunday. Senate Majority Leader Mitch McConnell promised to address Democrats' concerns over key issues such as immigration reform in a speech to the chamber on Sunday night.
Senate Minority Leader Chuck Schumer, a Democrat, said in response that he was "happy to continue my discussion with the majority leader about reopening the government" but added that the two sides were "yet to reach an agreement on a path forward."
‘Democrats are playing games’
Meanwhile, White House Press Secretary Sarah Sanders on Monday morning accused the Democrats of "playing games."
"The president's focus is making sure we get the government reopened. It's outrageous that Democrats are holding our national security hostage," she told ABC News.
Over the weekend, Trump called on Republicans in Congress to resort to the so-called nuclear option in order to end the government shutdown.
The nuclear option, which requires the Senate majority leader to change the parliamentary rules, means Republicans would no longer need 60 votes to pass legislation and can do so with a simple majority.
Despite having a 51-49 majority in the Senate, Republicans have had a hard time delivering on Trump’s campaign promises due to resistance from Democrats and a deep-running division within the party over whether they should distance themselves from the businessman-turned-president.
The GOP’s failure to line up behind Trump and his budget bill, which asks for a dramatic increase in military funding and border security, coupled with strong demands from Democrats for protecting immigrants, led to the impasse.