Nearly 250 refugees and migrants are still missing after two boats sank in the Mediterranean Sea over the weekend, bringing the death toll so far this year to more than 1,300 people, according to the UNHCR.

Survivors have been taken to Italy, which expects to take in 200,000 refugees over the course of this year.

The two shipwrecks took place on Friday and Sunday, but details did not emerge until later.

While Libya is the main gateway for refugees and migrants attempting to reach Europe by sea, the UN estimates that more than 49,000 people have crossed the Mediterranean so far this year, either landing in Italy, Greece or elsewhere.

READ MORE: African migrants traded in Libya's 'slave markets'

On Monday, the International Criminal Court prosecutor expressed alarm at the inhumane detention of thousands of vulnerable refugees and migrants in Libya and said she was examining whether an investigation could be opened into crimes against them.

According to the International Organization for Migration, 20,000 refugees and migrants are held by criminal gangs in irregular detention centres in Libya and growing numbers of migrants are traded in what they call slave markets before being held for ransom, forced labour or sexual exploitation.

Fatou Bensouda, the International Criminal Court (ICC) prosecutor, has told the UN Security Council that her office was collecting and analysing information "related to serious and widespread crimes allegedly committed against migrants attempting to transit through Libya".