U.S. President Donald Trump on Sunday signed a massive COVID-19 relief and spending package bill into law, the White House said.

The $2.3 trillion pandemic aid and spending package, which includes $1.4 trillion in spending to fund government agencies, will extend benefits to millions of Americans struggling with the pandemic and also avert a partial federal government shutdown.

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.

"As President, I have told Congress that I want far less wasteful spending and more money going to the American people in the form of $2,000 checks per adult and $600 per child," a statement from the president said after the signing. "Much more money is coming. I will never give up my fight for the American people!"

It's not immediately clear why the president reversed course on the deal, but doing so before a Monday night deadline avoided the government shutdown.

Trump had been criticized over the weekend for initially refusing to sign the much-needed COVID-19 relief package which was overwhelmingly passed by Congress following months of negotiations. 

He has repeatedly demanded Congress change the bill to increase the size of stimulus checks for struggling Americans to $2,000 from $600 and blasted the relief bill as "a disgrace."

Democrats are on board with the $2,000 payments but some Republicans have opposed them. Many economists agree the financial aid in the bill should be higher to get the economy moving again but say that immediate support for Americans hit by coronavirus lockdowns is still urgently needed.

President-elect Joe Biden, due to be sworn in on January 20 after beating Trump in November's election, had warned of "devastating consequences" on Saturday if the president continued his refusal to sign the bill.

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, without offering further information.

(With input from agencies)