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A vessel with around 400 people on board is adrift between Greece and Malta and is taking on water, support service Alarm Phone said on Sunday, after 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, overnight and that they had informed authorities. But authorities had not launched a rescue operation so far, they added.

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

German NGO Sea-Watch International said on its Twitter account it had found the boat with two merchant ships nearby.

It said the Maltese authorities had ordered the ships not to carry out a rescue and that one of them was just asked to supply it with fuel.

It was not immediately possible to reach Maltese authorities for comment.

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 said.

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

It said on Twitter the NGO found 25 people in the water during a rescue operation, and its staff were able to recover 22 survivors and two bodies, although it 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.

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.Ìý

(Reuters)