"The death of 260 people in less than ten days, in the most horrifying of circumstances, is evidence that the Mediterranean crisis is intensifying," Guterres said in a statement.
"Europeans need to take urgent action to stop this catastrophe getting worse in the second half of 2014," the UNHCR chief said.
Italian and Maltese naval crews last Saturday discovered 30 bodies - including that of a baby - who had apparently been overcome by heat and engine fumes after being locked in the hold of a boat carrying over 560 mostly-Syrian migrants.
A further 110 people - a figure revised up from 60 reported earlier this week - are believed to have been thrown into the ocean by their fellow passengers on the horror crossing from Tunisia.
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That tragedy and others took the number of people dead or reported missing during attempted crossing over the past 10 days to 260.
All-told, UNHCR said that 800 people have died this year alone in the Mediterranean. The grim tally compares with a total of 600 deaths in 2013, and 500 in 2012.
UNHCR said that 75,000 refugees and migrants arrived in Italy, Greece, Spain and Malta by sea in the first half of 2014.
That marked a 25-percent rise on the 60,000 who made the same journey in the whole of 2013, and over three times the 22,500 who arrived in all of 2012.
Many have fled the war in Syria or repression in Eritrea, preferring to risk their lives at the hands of people smugglers.
There has been a sharp rise in migrant landings in recent weeks because of the calm summer weather and growing lawlessness in Libya.
As in recent years, Italy continued to receive the greatest number of arrivals in the first six months of the year (63,884), followed by Greece (10,080), Spain (1,000) and Malta (227), UNHCR said.
A further 21,000 refugees and migrants have arrived in Italy since July 1st.
The Italian navy launched operation "Mare Nostrum" ("Our Sea") in October after 360 illegal migrants drowned off the island of Lampedusa in a disaster that shocked Europe and beyond.
Over the weekend of July 19-20 alone, UNHCR said, Italian and Maltese authorities, plus commercial vessels, rescued 8,000 people.
The European Union's Mediterranean members complain they are being abandoned by their partners in the 28-nation bloc to deal with the massive influx. Northern countries insist they have their own migrants to deal with.
Guterres called on all European governments to strengthen rescue operations, provide swift access to asylum procedures for those in need of protection, and legal alternatives to dangerous sea crossings.