The UNHCR issued its strongly-worded statement after more than 300 migrants were feared drowned this week when their overcrowded dinghies sank on the perilous crossing from north Africa to Europe.
It joins rights group Amnesty International and the International Organisation for Migration (IOM) in pushing for greater EU action to save migrants.
"UNCHR is today calling on the European Union to urgently change its approach to dealing with sea crossings with a view to ensuring that saving lives is the top-most priority," the Geneva-based agency said.
Italy decided to scale back the mission after its EU partners refused to share running costs of around nine million euros (USD 10 million) a month.
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Triton, which comes under the authority of the EU borders agency Frontex, has a monthly budget of 2.9 million euros (USD 3.3 million) and its patrols are generally restricted to the territorial waters of EU member states.
"There can be no doubt left after this week's events that Europe's Operation Triton is a woefully inadequate replacement for Italy's Mare Nostrum," said UN High Commissioner for Refugees Antonio Guterres.
Guterres said he is concerned that Europe is planning to phase out rather than step up its rescue efforts in response to the growing boat disasters.
Amnesty International slammed EU countries Wednesday for "burying their heads in the sand while hundreds keep dying at sea".
IOM spokesman Joel Millman told AFP his organisation does not "think it's wrong for countries and for Europe as a group to enforce border security".
But he said that Triton needs to devote more resources to its search and rescue operations and broaden their scope.
"There should be much more emphasis on legal migration.