Human Rights Watch says security forces are ignoring rights in dealing with threats.
The battle over Benghazi is part of a wider conflict involving two major factions.
Investigators have found that the co-pilot took over just before the plane descended sharply.
An audio message sets the new deadline for 29 January, by sunset, at the Turkish border.
A blizzard swept across the north-eastern United States, dropping more than 30cm of snow.
Militants claiming ties with Islamic State have taken responsibility for the attack.