A statement from the agency said survivors coming ashore today had reported that another rubber boat was unaccounted-for, likely raising the toll.
The UN refugee agency spokeswoman in Italy, Carlotta Sami, said at least 203 people had been "swallowed up by the waves."
Earlier in the week, the Italian coast guard reported that at least 29 people had died from hypothermia while travelling from north Africa to Italy's coast.
UNHCR blasted the new EU-backed rescue patrol as ineffective for saving lives. The European Union took over Mediterranean patrols after Italy phased out its robust Mare Nostrum operation, launched after 360 migrants died in 2013.