More than 700 people are feared to have drowned in the Mediterranean last week, making it the deadliest seven days for Europe-bound asylum seekers in more than a year.

Migrant boats capsized in three separate incidents last Wednesday, Thursday and Friday after more than 13,000 people set sail from Libya for Italy in an eight-day period.

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The vessel that sank on Thursday – a single wooden fishing boat being towed by a smugglers' boat from the Libyan port of Sabratha – had between 400 and 550 people on board, survivors said.

Humanitarian organisations say many boats sink without a trace, making it difficult to establish the exact number of dead.

A member of the German NGO Sea Watch described the scenes at sea as "gruesome”. Giorgia Liardi was quoted by the Guardian as saying: “There were already many dead bodies floating in the sea. Some of them were between life and death because they weren’t reacting, but still breathing.”

Calmer waters and warmer weather increased migrants' attempts to reach Europe, with Italy's southern islands being the main destinations.

Carlotta Sami, spokeswoman in Italy for UNHCR, put the number of migrants and refugees missing in Thursday's incident at 550. She said 15 bodies were recovered, while 70 survivors were taken from the sea and 25 swam to the other boat, AP reported.

Save the Children said most of the people on board were from Eritrea and included many women and children.

Sami said an estimated 100 people were missing from a smugglers' boat that capsized on Wednesday.

In Friday's shipwreck, 135 people were rescued, while 45 bodies were recovered and an unknown numbers of migrants were still missing, she said.

In April 2015 a single ship sank with an estimated 800 people trapped on board, prompting EU leaders to gather in Brussels a few days later to deal with the migration crisis.

According to sources, the EU Commission plans to launch a communication on a "new migration compact" in early June aimed at tackling establishing deals with African countries to stem the flow of people to Italy.