President Donald Trump abandoned a threat to shut down the US government, signing off on a budget despite being "unhappy" with many of its provisions -- and warning he won't back anything similar ever again.

He later tweeted his desire to have a "line-item veto" over future bills that would allow him to remove parts he disagrees with -- a measure that would alter the balance of the government and was ruled unconstitutional by the Supreme Court in 1998.

A visibly aggrieved Trump capped another anarchic week by approving the $1.3 trillion deal passed by the Republican-controlled Congress, just hours after threatening to veto it.

He fumed that a "crazy" lawmaking process had produced a bill that "nobody read" -- but said he was signing it as a "matter of national security."

"There are a lot of things that I'm unhappy about in this bill," he said in a hastily arranged and meandering address.

"There are a lot of things that we shouldn't have had in this bill. But we were, in a sense, forced," he said. "But I say to Congress, I will never sign another bill like this again. I'm not going to do it again."

He later riffed on the same theme on Twitter, where he said: "To prevent this omnibus situation from ever happening again, I'm calling on Congress to give me a line-item veto for all govt spending bills!"

Congress passed a short-lived Line Item Veto Act in 1996, giving then president Bill Clinton the power to scratch items from spending bills as a means to rein in fiscal excesses.

The law was challenged and later struck down by the Supreme Court in a 6-3 ruling which found it unconstitutional because it enabled the president to amend the text of statutes, the task of the legislative branch.

Presidents retained the power of veto over bills but without the ability to pick and choose which elements to retain and which to cut. AFP