The European Union should take a "more sophisticated, more courageous and less callous approach" to migrants who often die trying to reach the EU in boats that are not seaworthy, U.N. human rights chief Zeid Ra'ad Al Hussein said on Monday.

"Europe is turning its back on some of the most vulnerable migrants in the world and risks turning the Mediterranean into a vast cemetery," he said, describing EU policy as "short-sighted, short-term political reactions pandering to the xenophobic populist movements that have poisoned public opinion on this issue."

EU president Donald Tusk, on the other hand, is to host an emergency summit on illegal immigration this Thursday, after the latest migrant boat tragedies in the Mediterranean. "I have decided to call an extraordinary European Council this Thursday to address situation in the Mediterranean," Tusk said on Twitter on Monday.

Only a day after Sunday's tragedy, in which more than 700 people are feared to have perished off Libya, another boat with 300 people onboard issued a distress call from the Mediterranean.

EU foreign affairs chief Federica Mogherini told European nations on Monday that they had "no more excuses" not to act on the migrant crisis. Mogherini said the 28-nation EU had "no more excuses" now not to come up with a common response to the migrant tide. "We need immediate action from the EU and the member states," she said.

German Chancellor Angela Merkel is among those who want Europe to find answers, according to her spokesman. Steffen Seibert told reporters on Monday that Merkel, "like millions of Germans", is appalled by Sunday's capsizing of a fishing boat off Libya in which more than 700 people are feared dead, a few days after hundreds died in another incident.

The victims had "started out -- desperate, perhaps also hopeful -- for Europe, and the fact they now have only found death on the way, that is a tragedy," he said.

"That this is taking place with sad regularity in the Mediterranean, that is a situation not worthy of Europe. A continent which feels committed to humanity must look for answers even when there are no easy answers," he added.

Italian Prime Minister Matteo Renzi, whose country is among those bearing the brunt of the flood of migrants, said Rome was studying the possibility of mounting "targeted interventions" against the Libya-based people smugglers behind the crossings.

Europe's southern shores have been swamped over the past two weeks with migrants fleeing war and hardship, mostly via Libya.

Another migrant boat carrying 300 people sinks in Mediterranean: IOM

About 700 migrants feared dead after boat capsizes off the Libyan coast