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President Donald Trump unlawfully ordered National Guard troops to Portland in Oregon, a federal judge has ruled in a legal setback to the administration's use of the military in American cities.

The ruling by US District Judge Karin Immergut is the first to permanently block Trump's use of military force to quell protests against immigration authorities. Trump is also attempting to do that in Democrat-led Los Angeles, Chicago and Washington DC. It replaces her interim order that had prevented the Portland deployment.

Immergut, a Trump appointee, rejected the administration's claim that protesters at an immigration detention facility were waging a rebellion that legally justified sending in troops.

Trump's attempts to use military force to tamp down unrest are a sharp break with longstanding but rarely tested norms against deploying troops on US soil.

The Trump administration is likely to appeal Friday's ruling, and the case could ultimately reach the US Supreme Court.

The City of Portland and Oregon Attorney General's Office sued in September, alleging the Trump administration was exaggerating occasional violence to justify sending in troops under a law permitting presidents to do so in cases of rebellion.

Duelling narratives emerged during three-day bench trial. Justice Department lawyers described a violent siege overwhelming federal agents, echoing Trump's description of the city as "war-ravaged".

"For months, agitators have used violence and threatened violence against the men and women who serve our country," attorney Eric Hamilton said at trial.

Lawyers for Oregon and Portland said violence has been rare, isolated and contained by local police.