Survivors of another deadly Mediterranean crossing reported that at least 200 migrants — and possibly many more — died in frigid temperatures in the open-sea crossing from Libya to Europe, the U.N. refugee agency said Wednesday.

The deaths add to the 29 reported earlier in the week by the Italian coast guard, which said the victims had died of hypothermia during the voyage that began Sunday in Libya.

The U.N. High Commissioner for Refugees (UNHCR) said survivors reported that at least a further 203 people traveling in three rubber boats had died. It quoted survivors as saying there was a fourth boat, and said if its existence is confirmed, the toll would rise.

The agency's spokeswoman in Italy, Carlotta Sami, said 203 people had been "swallowed up by the waves," the youngest aged 12.

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.

But the EU's Triton mission only operates a few miles off Italy's coast, whereas Mare Nostrum patrols took Italian rescue ships up close to Libya's coast, where most of the smuggling operations originate.

"The Triton operation doesn't have as its principal mandate saving human lives, and thus cannot be the response that is urgently needed," Laurens Jolles, the head of the U.N. agency for southern Europe, said in a statement.