President Donald Trump has threatened to close the southern US border unless Congress agrees to provide $5 billion in taxpayer funds for a border wall with Mexico.
A dispute over funding for Mr Trump's proposed wall has led to "non-essential" operations at numerous agencies being closed for lack of funding.
With Congress adjourned until next week, there was no prospect of a quick resolution.
"We will be forced to close the Southern Border entirely if the Obstructionist Democrats do not give us the money to finish the Wall & also change the ridiculous immigration laws that our Country is saddled with," Mr Trump tweeted.
"Either we build (finish) the Wall or we close the Border," he added.
The standoff over Mr Trump's demand for money to help fund the border wall, estimated to cost about $23 billion in total, was in its seventh day and was widely expected to drag into January when Democrats take control of the US House of Representatives.
Asked about Mr Trump's border-closing threat, Mexican President Andres Manuel Lopez Obrador told reporters it was an internal US government matter.
"We take great care of the relationship with the government of the United States," Mr Lopez Obrador said.
"Of course we will always defend our sovereignty ... We will always protect migrants, defend their human rights," he said.
Earlier this month, the Republican president said he would be "proud" to shut down the government over border security.
In television interviews today, Trump aides sought to blame Democrats for the continuing shutdown, contending that they have refused to negotiate since the White House made an offer last weekend.
"We're here, and they know where to find us," Mick Mulvaney, the White House chief of staff, said in an interview with Fox News Channel.
"Where is Chuck Schumer? Where is Nancy Pelosi? They're not even talking right now," he said.
Mr Schumer is the top Democrat in the Senate and Ms Pelosi is the incoming speaker of the House of Representatives.
A spokesman for Mr Schumer said the White House has been told that there are three existing federal funding proposals containing funds for border security that could pass both the House and the Senate - and that Mr Trump should accept one of those.
At present, however, the two sides remain far apart, Mr Schumer's office said.
Democrats have offered support for $1.3 billion in funding for general border security, but have long opposed the building of a wall.
Three options for federal funding
If the stalemate persists, House Democrats, led by Ms Pelosi, plan to immediately offer a funding measure to re-open the government when they take office on 3 January.
Ms Pelosi and Mr Schumer have been discussing three general options for that legislation, according to a senior Democratic aide.
The options are a stop-gap funding bill that would run through 8 February; six full 2019 appropriations bills for all but the Department of Homeland Security, which would instead be funded through a measure known as a continuing resolution maintaining current funding through 30 September; or a continuing resolution for all shuttered government agencies that would expire on 30 September.
The aide said the legislation expected for a full House vote on 3 January could be a variation on any of the three options but added that no decision has been made.
According to a Reuters/Ipsos poll released yesterday, 47% of Americans hold Mr Trump responsible for the shutdown, while 33% blame Democrats in Congress.
The shutdown affects about 800,000 employees of the Departments of Homeland Security, Justice, Agriculture, Commerce, and other agencies.
Most of the federal government, which directly employs almost four million people, is unaffected.
Even agencies that are affected never totally close, with workers deemed "essential" still performing their duties.
Mr Trump, who scrapped plans to spend Christmas in his Mar-a-Lago resort in Florida and stayed in Washington due to the shutdown, had now also cancelled his New Year's plans, Mr Mulvaney said.
