The "Bulwark" was first on the scene after the International Organisation for Migration received a distress call from a migrant boat that got into trouble off the coast of Libya.
The migrants were subsequently transferred to an Italian vessel. Their dinghy sank.
The Italian coastguard on Thursday also coordinated the rescue of another rickety boat carrying 120 migrants.
More than 5,000 migrants have died trying to reach Europe's shores in rickety boats over the past 18 months.