US President Donald Trump has signed a massive $1.5 trillion tax overhaul into law, while also approving a short-term spending bill that averts a possible US government shutdown.

The two pieces of Republican legislation represent Mr Trump's most significant accomplishment with Congress since taking office in January, as well as a sign of what awaits when he returns from his Mar-a-Lago resort in Florida after the Christmas holiday.

The tax package, the largest such overhaul since the 1980s, slashes the corporate tax rate from 35% to 21%, and temporarily reduces the tax burden for most individuals as well.

Democrats had opposed the bill as a giveaway to the wealthy that would add $1.5 trillion to the $20 trillion US national debt during the next decade.

The spending bill extends federal funding through 19 January largely at current levels.

It does nothing to resolve broader disputes over immigration, healthcare and military spending.

Republicans are also divided over whether to follow up their sweeping overhaul of the US tax code with a dramatic restructuring of federal benefit programs.

House Speaker Paul Ryan has said he would like to revamp welfare and health programs but Senate Republican Leader Mitch McConnell told NPR on Monday that he was not interested in cutting those programs without Democratic support.