Fourteen refugees have drowned in another boat sinking as European Union and African leaders meet to discuss measures to stem the flow of people into Europe.
Seven of those who died when a wooden boat sank between Turkey and the Greek island of Lesbos were children. Coastguards rescued 27 survivors.
The meeting in the Maltese capital Valletta was planned after about 800 died in a migrant boat sinking off Libya in April.
Almost 800,000 migrants have arrived in Europe by sea so far in 2015, while some 3440 have died or gone missing making the journey, the UN said.
Some 150,000 people from African countries such as Eritrea, Nigeria and Somalia have made the dangerous journey across the Mediterranean from Africa so far this year, arriving mainly in Italy and Malta.
But this has been dwarfed by the arrival of some 650,000 people - mostly Syrians - via Turkey and Greece.
The crisis has evolved so quickly this year that European leaders have struggled to keep up and formulate any coherent policies.
At the two-day Malta summit, EU leaders are expected to offer countries in Africa billions of euros in exchange for help with the refugee crisis.
The European Commission is setting up a €1.8bn ($NZ2.95bn) "trust fund" for Africa and has urged member states to match that sum. However, there are doubts about whether they will do so.
The aim is to tackle the economic and security problems that cause people to flee, and persuade African countries to take back more failed asylum seekers.
African leaders are likely to insist on a much clearer path for smaller numbers of their citizens to migrate officially to Europe, in exchange for help on the crisis.
Tensions in the EU have been rising because of the pressures faced by those countries where most migrants initially arrive, particularly Greece, Italy and Hungary. Most migrants then head to Germany or Sweden - regarded as the most welcoming to refugees - to claim asylum.
EU leaders have agreed a controversial programme to relocate thousands of migrants - but so far only about 130 have been successfully moved from Greece and Italy.
Wednesday evening saw Swedish Interior Minister Anders Ygeman announce that his country will impose temporary border controls from noon on Thursday local time for 10 days until 21 November to allow it to cope with tens of thousands of new arrivals.
"We moved forward with temporary border controls in order to obtain security and stability... not to limit the number of asylum seekers, but to get better control of the flow of asylum seekers to Sweden," he said.
Other developments highlighted deep divisions between EU members on the migrant issue.
Hungary was bitterly critical of Germany's announcement that it planned to send more Syrian refugees back to the first EU country they had entered, after reinstating the EU's Dublin Regulation on asylum. Berlin said this would not include Greece, the first point of entry for most migrants.
Hungarian Foreign Minister Peter Szijjarto said: "The Dublin system is dead."
He also accused German politicians of making "irresponsible statements" which some migrants had interpreted as an invitation to come to Europe.
Meanwhile, Slovenia has begun building a razor-wire fence along its border with Croatia, a day after the government said it would install "temporary technical obstacles", but stressed official border crossings would remain open and that the move was designed to restore order.
Croatia criticised the Slovenian fence, saying it would be better to spend money on preparing reception centres for migrants.