Washington: US President Donald Trump on Tuesday pardoned two imprisoned Oregon ranchers whose sentencing on arson convictions sparked the 2016 occupation of a wildlife refuge, part of a long-simmering dispute over federal land policies in the US West.

The decision by Trump prompted dismay by conservation groups who called the Hammonds "lawless extremists".

Rancher Dwight Hammond jnr in 2016. Credit:AP

The armed standoff at the Malheur National Wildlife Refuge in remote southeast Oregon followed a judge's ruling sending Dwight Hammond and his son Steven back to prison to serve longer terms after their initial release from shorter sentences.

Police shot one of the occupiers dead during the 41-day mid-winter protest.