US President Donald Trump has signed into law a $US2.3 trillion ($A3trillion) pandemic aid and spending package, restoring unemployment benefits to millions of Americans and averting a federal government shutdown.

Trump, who leaves office on January 20 after losing November’s election to Joe Biden, backed down on Sunday from his threat to block the bill, which was approved by Congress last week.

He’d come under intense pressure from both sides of politics.

Trump golfed on Sunday and remained out of public view even as the potential government crisis loomed.

Tweet from @DanRather

He’d demanded Congress change the bill to increase the size of stimulus cheques for struggling Americans to $US2000 from $US600.

It was not immediately clear why he changed his mind as his resistance to the massive legislative package promised a chaotic final stretch of his presidency.

White House officials have been tight-lipped about Trump’s thinking but a source familiar with the situation said some advisers had urged him to relent because they did not see the point of refusing.

“Good news on Covid Relief Bill. Information to follow!” Trump said in a cryptic message on Twitter earlier on Sunday evening. But he offered no explanation.

Democrats are on board with the $US2000 payments but many Republicans have opposed it in the past.

Tweet from @SpeakerPelosi

Many economists agree the financial aid in the bill should be higher to get the economy moving again but say immediate support for Americans hit by coronavirus lockdowns is still urgently needed.

Unemployment benefits being paid out to about 14 million people through pandemic programs lapsed on Saturday but will be restarted now that Trump has signed the bill.

The package includes $US1.4 trillion in spending to fund government agencies.

If Trump had not signed the legislation, then a partial government shutdown would have begun on Tuesday that would have put millions of government workers’ incomes at risk.

Global share prices ticked up in response to the news.

Trump’s abrupt move came after most Republicans refused to back his call for changes to legislation they had already voted on.

Hours earlier, Senator Pat Toomey of Pennsylvania told Fox News Trump wants “to be remembered for advocating for big cheques but the danger is he’ll be remembered for chaos and misery and erratic behavior if he allows this to expire”.

Trump sought to put the best face on his climb-down, saying he was signing the bill with “a strong message that makes clear to Congress that wasteful items need to be removed”.

He noted Democratic-controlled House of Representatives planned to vote on Monday to increase relief cheques to individuals from $US600 to $US2000, and said the Senate “will start the process” to approve higher payments.

Senate Majority Leader Mitch McConnell said “I thank the president” but made no mention of any plans for a Senate vote on higher relief payments.

Trump spent the Christmas holiday at his Mar-a-Lago resort in Florida.

-AAP