A 28-year prison sentence is too lenient for a Libyan militant who was convicted of terrorism-related charges in the 2012 attacks, a federal appeals court ruled Friday.

A federal appeals court has concluded that a 28-year prison sentence is too lenient for a Libyan militant who was convicted of terrorism-related charges in the 2012 attacks on U.S. compounds in Benghazi, Libya, that killed Ambassador Chris Stevens and...