A smuggler’s boat crammed with hundreds of people overturned off Libya’s coast as rescuers approached, causing what could be the Mediterranean’s deadliest known migrant tragedy and intensifying pressure on the European Union Sunday to finally meet demands for decisive action. Italian Premier Matteo Renzi said his authorities were “not in a position to confirm or verify” how many were on board when the boat set out from Libya. Eighteen ships joined the rescue effort, but only 28 survivors and 24 bodies had been pulled from the water by nightfall. With yesterday’s tragedy, demands for decisive action were going mainstream as authorities from France, Spain, Germany and Britain joined calls for a unified response.