"Europe is living through a maritime refugee crisis of historic proportions," the agency warned in a report.
The numbers flooding across the Mediterranean, often in rickety boats and at the mercy of human traffickers, have swelled 83 percent compared to the first six months of 2014, when 75,000 people made the journey, it said.
The situation is expected to deteriorate further as more clement summer weather allows ruthless people smugglers to dispatch more people.
Arrivals in the second half of 2014 were for instance nearly double those of the first half, UNHCR pointed out.
The immigration crisis is a burning issue for the EU, where member states have been wrangling over the best ways to tackle human trafficking and arguing over how to share the burden of helping new arrivals, many of them ill, starving and destitute.
The soaring numbers arriving in Italy and Greece, before moving on to other northern European states in the hope of finding jobs, has sparked outcry and growing anti-foreigner rhetoric in many countries.
The report hailed Brussel's decision to distribute 40,000 Syrian and Eritrean asylum-seekers who have already arrived in Europe among EU members but called for greater solidarity between countries — to help both the migrants and the states worst affected by the crisis.
Most not economic migrants
UN refugee chief Antonio Guterres stressed most of those attempting the dangerous journey across the Mediterranean are not economic migrants.
"Most of the people arriving by sea in Europe are refugees, seeking protection from war and persecution," he said in a statement.
A third of those who have arrived by sea in Italy or Greece this year came from war-ravaged Syria, while people fleeing violence in Afghanistan and Eritrea's repressive regime each made up 12 percent of arrivals.
Other top countries of origin include conflict-wracked Somalia, Nigeria, Iraq and Sudan, the report said.
This year has also seen a sharp increase in the numbers of people dying as they try to cross the Mediterranean.
So far 1,867 have been killed — 1,308 of them in April alone.
The unprecedented number of deaths that month spurred European leaders to significantly broaden search and rescue operations in the Mediterranean, cutting fatalities to 68 in May and 12 in June.
"With the right policy, backed by an effective operational response, it is possible to save more lives at sea," Guterres said.