Both tragedies were a result of a renewed surge in migrants fleeing war and poverty that has confronted Europe with its worst refugee crisis since World War Two.
The International Organization for Migration said it estimated a third of a million people had crossed the Mediterranean so far this year to wash up in southern Europe.
Almost two-thirds had arrived in Greece and most of the rest in Italy. At least 2,636 had perished in the attempt.
German Chancellor Angela Merkel said European Union leaders were ready for an emergency meeting, if necessary, to discuss the refugee crisis. The White House urged Europe to crack down on traffickers and ensure that migrants' human rights were protected.
In the latest disaster off the coast of North Africa, a vessel packed with an estimated 400 migrants sank on Thursday after leaving Zuwara in Libya. The port is a major launchpad for smugglers exploiting a security vacuum in a country with two rival governments.
Lacking navy ships, Libyan officials were searching for survivors with fishing boats and inflatables provided by locals. About 198 people had been rescued by noon, officials said.
"The boat was in a bad condition and people died with us," said Ayman Talaal, a Syrian survivor, standing next to his daughter. "We have been forced into this route. It's now called the grave of the Mediterranean Sea."
The migrants were from sub-Saharan Africa, Pakistan, Syria, Morocco and Bangladesh, a security official said. Many appeared to have been trapped in the hold when the boat capsized.
Local officials and residents were putting bodies into red bags on a beach littered with shoes, trousers and other personal items from drowned migrants.