German migrant rescue charity Sea-Eye said Friday one of its boats had picked up 223 migrants in the Mediterranean and accused Malta of failing to respond to distress calls.

The Sea-Eye 4 rescue ship has picked up the migrants in four rescue missions since Thursday "in the Maltese search and rescue zone", the NGO said in a statement.

"Among them are 29 women, four of whom are pregnant, and eight children. The crew is currently searching for another boat in distress," it said.

"Malta has again failed to fulfil its obligation to coordinate and solve maritime emergencies," it added.

According to Sea-Eye, civilian sea rescue organisations are currently the only European forces actively searching for people and willing to bring them to safety.

"As the weather conditions are soon expected to worsen considerably, the chances of survival for those still at sea are decreasing considerably," the charity said.

One child on board has a broken arm and another a broken finger, while two pregnant women are suffering from stomach pains, it said 

Several people have had to be treated for chemical burns and hypothermia, it added.

Every year, tens of thousands of migrants seek to cross the central Mediterranean from north Africa, mainly from Libya and Tunisia.

The perilous crossings are often deadly, and the journeys are made even more complicated by winter weather.