Italy's ANSA news agency said an estimated 700 people were aboard and only 28 people had been rescued.

Barbara Molinario, spokeswoman for the United Nations refugee agency, said the Italian Coast Guard operation is continuing and the number of victims is not known.

"It's clear that a boat overturned and there are people missing, but on numbers (dead or alive) it's too soon to tell," she said.

The capsizing comes amid a wave of migrants trying to leave Libya for Italian shores. They seek to take advantage of calmer seas and warm weather to make the dangerous crossing on smugglers' boats.

So far, at least 900 have died trying this year as the overcrowded boats capsize.

If anywhere close to 700 people are confirmed to have died, this would be the deadliest toll from a single capsizing. Just last week, 400 people were presumed drowned when another boat capsized.

On Saturday, Pope Francis joined Italy in pressing the European Union to do more to help the country cope with the soaring numbers of arrivals, which include more than 10,000 in the week ending Saturday.

Italy says it will continue rescuing migrants but demands that the European Union increase assistance to shelter and rescue them. Italy says the EU's border control agency, Frontex, must take a greater role in coordinating rescue operations. Frontex technically is a border patrol operation, not responsible for search and rescue efforts.

Because most migrants want to reach family or other members of their community in northern Europe, Italian governments have pushed for those countries to do more, particularly by taking in the migrants while their requests for asylum or refugee status are examined.

The European Union's commissioner for migration, Dmitris Avramopoulos, says a new policy will be presented in May. He also has called for other EU members to provide more aid to Italy.