"Thus far 2016 is proving to be particularly deadly," said William Spindler, spokesman for the agency, in announcing the soaring death toll in Geneva Tuesday.
He said 2,510 people have died making the perilous crossing so far this year, compared to 1,855 in the first five months of 2015 -- an increase of 35%. Fewer than 60 died in the same period in 2014.
At least 880 migrants drowned in a series of sinkings and wrecks in the past week, said Spindler, citing new information from survivors who reached Italy.
He said that on top of three major sinkings of overcrowded vessels from Libya
that occurred Wednesday
and Friday, officials had subsequently learned that 46 were missing from a raft carrying 125 people that deflated, eight had been lost overboard from another and four died in a fire on board another.
Most of the migrants in the recent wrecks were from Somalia, Eritrea and Sudan, the U.N refugee agency said last week.
Central Mediterranean route 'more dangerous'
Spindler said that the central Mediterranean route -- from Libya or other north African countries to Italy -- was "dramatically more dangerous" than the Turkey-Greece route taken by most migrants.
Boats on the central Mediterranean route were more crowded than those typically seen on the Turkey-Greece route, often carrying 600 or more passengers, he said.
The boats, the majority of which set sail from the Sabratah area west of Libya's capital, Tripoli, were sometimes towed by larger fishing boats, which were also put at risk, he said.
More than 2,100 of the migrant deaths at sea so far this year have happened on the central Mediterranean route, making the odds of dying on the journey one in 23.
Less than a quarter of migrants who have made the journey into Europe by sea so far this year took the central Mediterranean route -- 46,714, compared with 156,364 entering Greece by sea.
Across the Mediterranean as a whole, the odds of dying while attempting to illegally enter Europe by sea were one in 81, Spindler said.
Shift in migration patterns
The latest spate of deaths at sea come on the heels of a dramatic recent shift in the migration patterns across the Mediterranean into Europe, according to the International Organization for Migration (IOM).
IOM figures released last week
showed a spike in the numbers of migrants entering Europe via Italy in the two weeks ending May 18 -- up 54% compared with the previous two weeks. Meanwhile, it said, migration to Greece -- the main entry point for the bulk of illegal migrants into the European Union -- had dropped 67% over the same period.
The IOM attributed the change to a deal between Turkey and the European Union to try to control migration into Greece, which it said has "brought migration flows through the Eastern Mediterranean and the Western Balkan route to a standstill."
But Spindler said that the U.N. Refugee Agency had not seen evidence of "a significant diversion of Syrians, Afghans or Iraqis from the Turkey-Greece route to the Central Mediterranean one."
"The principal nationalities on the Libya to Italy route so far this year have been Nigerians and Gambians, although among countries more commonly associated with refugee movements, 9% have been Somalis and 8% Eritreans," he said.
Many Somalis are trying to escape Al-Shabaab and clan warfare, while Eritreans seek to flee government repression or military service.
Spindler said that survivors reported that smuggler hubs in locations including Niger were transporting people from West Africa to Libya, where many stayed for months before attempting the voyage to Europe.
Reports of sexual and other forms of violence against women on the journey were common, he said.
"Some women have told us they were subject to sexual slavery in Libya," he said.
U.N. officials were also seeing an increase in the numbers of unaccompanied children arriving, he said.
Last year, 3,771 people died on the Mediterranean trying to enter Europe, according to figures from the U.N. Refugee Agency.
ISIS is trying to infiltrate the migrant routes from Libya
to smuggle its jihadists into Europe as the route from Turkey to Greece becomes more heavily policed, intelligence officials have told CNN.