US President Donald Trump lashed out in unusually personal terms against the six Supreme Court justices who handed him one of the biggest setbacks of his second term in office by striking down the administration's global tariffs.

The court's Friday ruling was "deeply disappointing". The justices who joined the majority opinion should be "absolutely ashamed" and lacked the courage to "do the right thing", Trump said, turning his response into a sweeping attack against a co-equal branch of government.

The broadside was remarkable even for a president known for blowing past political norms and publicly berating anyone who challenges his authority.

"I'm ashamed of certain members of the court. Absolutely ashamed for not having the courage to do what's right for our country," Trump said at the start of a press conference at the White House, which was held a few hours after the decision was released.

Trump did not mince words from there as he assessed the decision, which held that presidents do not have inherent authority to impose sweeping tariffs on any country.

For the next 45 minutes, Trump criticised the ruling and made the case that he would find other methods to continue imposing tariffs on other countries. But throughout he repeatedly returned to the justices in ways that made clear he felt personally slighted by the decision.

The president did not discriminate against Republican and Democratic appointees, either.

The six justices who struck down Trump's tariffs were equally divided among the court's liberal and conservative wings. Three - Justices Elena Kagan, Sonia Sotomayor and Ketanji Brown Jackson - were appointed by Democratic presidents. The other three were appointed by Republicans. Chief Justice John Roberts is a George W Bush pick who wrote the majority opinion, and Justices Neil Gorsuch and Amy Coney Barrett were appointed by Trump in his first term in office.

Trump went after them all.

"They're just being fools and lapdogs for the Rhinos and the radical left Democrats," Trump said, using shorthand for a term - Republicans in Name Only - that is deployed by some on the right to disparage other Republicans deemed not sufficiently loyal to the party.

The president also claimed that the court was influenced by "foreign interests" in its decision, though he did not provide any details or evidence for the assertion.

"It's my opinion that the court has been swayed by foreign interests," he said.

Trump declined to elaborate when a reporter pressed him to explain what he meant.

When asked if he regretted nominating Gorsuch and Coney Barrett, the president stopped short of saying that he had made a mistake. But Trump said their votes were an "embarrassment" and brought up their families, a highly unusual move.

"It's an embarrassment to their families, to one another," Trump said.

At the same time, Trump heaped praise on the three members of the court, Justices Clarence Thomas, Samuel Alito, and Brett Kavanaugh, who voted to keep his tariffs authority intact.

He gave special attention to Kavanaugh, his other first-term appointee.

In a lengthy dissent, Kavanaugh said the government would be forced to refund billions in tariff revenue and said the process would be a "mess." The president thanked Kavanaugh, as well as Thomas and Alito, "for their strength and wisdom and love of our country".

Court watchers and trade experts said Trump's reaction wasn't surprising given how much he had invested in the outcome of the case.

"I think the court was well aware of the importance to the president of this decision," said Alan Wm Wolff, a former deputy director-general of the World Trade Organization.

Colin Grabow, a trade expert at the Cato Institute, a libertarian think tank in Washington, said the decision represented "a victory for the rule of law".

"It's unfortunate that he attacked these justices," Grabow said.

"The Supreme Court said [Trump] went too far," he added. "President Trump took that as an affront. It's not a surprise."