South Korea’s jailed former president Yoon Suk-yeol had his prison sentence increased on Wednesday in a separate case linked to his failed martial law decree, in what analysts said could be a “bellwether” for the trials still unfolding from the crisis.

The Seoul High Court raised Yoon’s sentence from five years to seven for obstruction of justice and other offences after finding that he used presidential security agents to block investigators trying to arrest him over the December 2024...