The Department of Homeland Security (DHS) entered a partial shutdown on Saturday as US lawmakers fight over funding the agency overseeing much of President Donald Trump's immigration crackdown.

Thousands of government workers, from airport security agents to disaster relief officials, will either be furloughed or forced to work without pay until funding is agreed upon by Congress.

At the centre of the budget dispute is the department's Immigration and Customs Enforcement (ICE), whose agents killed two US citizens amid sweeping raids and mass protests in Minneapolis.

Democrats oppose any new funding for DHS until major changes are implemented over how ICE conducts its operations.

In particular, they have demanded curtailed patrols, a ban on ICE agents wearing face masks during operations and the requirement that they obtain a judicial warrant to enter private property.

"Donald Trump and Republicans have decided that they have zero interest in getting ICE under control," House Minority Leader Hakeem Jeffries said on Friday.

"Dramatic changes are needed," he said. "Absent that, Republicans have decided to shut down parts of the federal government."

But while DHS faces a shutdown, ICE itself will remain operational, under funds approved in last year's government spending bill.

Senator John Fetterman pushed against his fellow Democrats, saying: "This shutdown literally has zero impact on ICE."

The primary impact would land on other agencies, including the Federal Emergency Management Agency, which oversees emergency response to natural disasters.

The Transportation Security Administration, which runs airport safety, warned on X that a prolonged shutdown could result in longer wait times and cancelled flights.

The shutdown would be the third of Trump's second term, including a record 43-day government closure in October and November.

The government just reopened from a smaller, four-day partial shutdown this month, also over DHS funding. (AFP)