The German-flagged refugee rescue vessel Louise Michel needed reinforcements after lending help to a boat in the Mediterranean Sea that was carrying at least one dead migrant.
The rescue boat became overcrowded and unable to move after encountering another boat attempting to cross the expanse dividing Europe and Africa with 130 people on board.
"There is already one dead person on the boat. We need immediate assistance," the crew of the 31-meter (101 feet) Louise Michel wrote on Twitter, saying other migrants had fuel burns and had been at sea for days.
The vessel's crew of 10 had earlier rescued another 89 people from a rubber boat in distress on Thursday, and said European rescue agencies had so far ignored its distress calls.
The boat — named after 19th-century French anarchist Louise Michel — was around 90 kilometers southeast of the Italian island of Lampedusa early on Saturday, according to the global ship tracking website Marine Traffic.
Thousands of people are thought to have died making the dangerous trip across the Mediterranean to flee conflict, repression, and poverty in Africa and the Middle East.
Banksy's decision to fund the high-speed boat follows a body of work by the artist that has leveled scathing judgments on Europe's halting response to the migrant crisis.