MILAN — Shaken by the feared drowning of hundreds of people in the latest Mediterranean tragedy, European leaders struggled Monday for an adequate response in the face of unremitting migrant flows and continued instability in Libya that has given free rein to human traffickers. Even as the search continued for victims of the weekend disaster that left as many as 950 people feared dead, coast guard ships rushed to respond to new distress calls on the high seas – two off Libya and a third boat that ran aground near Greece. Decrying what he called an “escalation in these death voyages,” Italian Premier Matteo Renzi urged Europe to put the focus on preventing more boats from leaving Libya, the source of 90 percent of migrant traffic to Italy. “We are facing an organized criminal activity that is making lots of money, but above all ruining many lives,” Renzi said at a joint news conference with Malta’s prime minister, Joseph Muscat.