More than thirty people attempted the crossing on the vessel, with many of the survivors in an “extremely serious” state of health according to UNHCR.
“Six Syrian refugees including children, women and teenagers lost their lives at sea. They died of thirst, hunger and severe burns,” Chiara Cardoletti, UNHCR official in Italy, wrote on her Twitter account.
“This is unacceptable. Strengthening rescue at sea is the only way to prevent these tragedies,” she added.
The victims included two children aged one and two years old, a 12-year-old child and three more adults, including a grandmother and mother also traveling with children who survived the journey, according to a UNHCR statement.
The 26 survivors were being treated in Pozzallo, Sicily.
It was not specified which country they had departed from.
Alarm Phone, a group running a hotline for migrants needing rescue, said some 250 migrants were in need of rescue off the coast of Malta.
The boat left Lebanon around a week ago, it said, and had now run out of fuel.
“Food & drinking water finished 2 days ago. The caller said his 3-month old daughter has died from thirst,” the group posted on Twitter.
Several hundred migrants have been rescued in the past few days in the Mediterranean by humanitarian groups.
German NGO SeaWatch said on Monday it had 428 people on board a ship, waiting for a port to be assigned.
“The civil fleet has rescued hundreds of people in the past days, who otherwise would have been pulled back to Libya or even drowned,” it said on Twitter.
According to the European border and coast guard agency Frontex, the central Mediterranean route was used by more than 42,500 migrants from January to July an increase of 44 percent compared to the first seven months of 2021.
UNHCR says 1,200 people have lost their lives in the Mediterranean since the beginning of the year.