A US appeals court has ruled that most tariffs issued by US President Donald Trump are illegal,

A US appeals court has ruled that most tariffs issued by US President Donald Trump are illegal, setting up a potential legal showdown that could upend his foreign policy agenda.

The ruling affects Trump's "reciprocal" tariffs, imposed on most countries around the world, as well as other tariffs slapped on China, Mexico and Canada.

But, the ruling will not take effect until 14 October to give the administration time to ask the Supreme Court to take up the case.

Trump criticised the appeals court and its ruling on Truth Social, saying: "If allowed to stand, this Decision would literally destroy the United States of America."