A vessel with around 400 people on board is adrift between Malta and Libya and is taking on water, support service Alarm Phone said on Sunday, amid a sharp rise of migrant boats crossing the Mediterranean from North Africa.

Alarm Phone said on Twitter they had received a call from the boat, which departed from Tobruk, in Libya, last night and that they had informed authorities. But no rescue operation has been launched so far, they added.

Alarm Phone said people on board were panicking, with several of them requiring medical attention. The vessel was out of fuel and its lower deck was full of water, while the captain had left and there was nobody who could steer the boat, they added.

Alarm Phone said the boat was now in the Maltese Search and Rescue area (SAR).

It was not immediately possible to reach Maltese authorities.

German NGO Sea-Watch International said on its Twitter account that on Sunday it was carrying out searches of boats in distress in the Mediterranean Sea, including the one flagged by Alarm Phone.

Another NGO, Germany's Resqship, said on Sunday at least 23 migrants died the previous night in the Mediterranean in a separate shipwreck.

They said on Twitter they found 25 people in the water during a rescue operation, and that they were able to recover 22 survivors and two bodies. But Resqship added their crew was told about 20 other people had already drowned.

Last week 440 migrants were rescued off Malta after a complex 11-hour operation in stormy seas by the Geo Barents vessel of the Doctors Without Borders (MSF) charity, while at least 23 African migrants were missing and four died on Saturday after their two boats sank off Tunisia as they tried to reach Italy.