The Trump administration is warning federal workers furloughed in the government shutdown that they may not get back pay once the funding impasse ends, which some lawmakers are decrying as illegal.

A memo prepared by the Office of Management and Budget says a 2019 law signed by President Donald Trump, guaranteeing retroactive pay after a shutdown, did not apply to employees who have been temporarily asked not to report to work.

On Tuesday, Trump told reporters that back pay "depends on who we're talking about" and that some workers "don't deserve to be taken care of".

About 750,000 employees are currently foregoing their regular pay as the shutdown nears the end of its first week.

The top Democrat in the US House of Representatives, Hakeem Jeffries, disagreed with Trump's interpretation.

"The law is clear — every single furloughed federal employee is entitled to back pay, period," he said on Tuesday.

The memo was prepared last week but was first reported by US media on Tuesday.

In it, the OMB said that once the shutdown ended the government was required to pay employees who were deemed essential, such as air traffic controllers, and had been required to show up to work.

But it said the government didn't have a duty to retroactively pay those who were furloughed, essentially sent home with no pay.

A 2019 law passed by Congress and signed by Trump during his first term "requires employees of the federal government or a District of Columbia public employer who are furloughed or required to work during a lapse in appropriations beginning on or after December 22, 2018, to be compensated for the period of the lapse."

The law was passed after the last major shutdown, which lasted more than a month.

The current shutdown began on 1 October. Republicans and Democrats in the Senate are currently locked in a standoff, each putting forward resolutions for reopening the government that have failed in multiple rounds of voting. Democrats want any resolution to address health care, while Republicans want "clean" legislation that only deals with the federal budget.

Republican House Speaker Mike Johnson told a news conference on Tuesday that he had not yet spoken to the White House about back pay, but new legal analysis was challenging the requirement to retroactively pay furloughed employees.

"It's true that in previous shutdowns, many or most of them have been paid for the time they were furloughed," he said. "There are some legal analysts who are saying that may not be appropriate or necessary."

If that analysis turned out to be valid, he added, then it would put pressure on Democrats to join Republicans and pass their resolution to reopen the government.

Congressional records show Johnson voted for the 2019 law.

The OMB memo was roundly criticised by Democratic lawmakers.

"This is totally illegal," Senator Mark Warner of Virginia said.

He called the memo "another illegitimate attempt" by OMB Director Russ Vought to "traumatize" federal workers, "which seems to be what gets him out of bed in the morning".

Some Republicans also seemed to sceptical of Trump's approach.

South Dakota's John Thune, the top Republican in the Senate said he expected furloughed workers to be paid.

Another Republican, North Carolina Senator Thom Tillis, said it was a "strategic mistake" to let furloughed workers "know, or let them think, that they could potentially not get back pay".

"I don't think that that is a helpful discussion right now," he said.