At least 240 refugees and migrants are feared to have died after two boats capsized in the Mediterranean.
Proactiva Open Arms pulled five bodies from the waters off the Libyan coast after at least two rubber ships sunk into the sea.
But the Spanish aid organization believes hundreds more could be drowned because each giant dingy is often packed with as many as 120 people.
“We brought on board five corpses recovered from the sea, but no lives. It is a harsh reality check of the suffering here that is invisible in Europe,” the group wrote on Facebook.
Numbers of migrants trying to reach Europe from Libya via Italy have risen dramatically this year since the route between Turkey and Greece was effectively shut down.
The Italian coast guard said they had co-ordinated more than 40 rescue operations in the last few days.
The International Organization for Migration (IOM) says more than 20,000 migrants have arrived in Italy so far this year – and some 559 people are estimated to have died or gone missing en route.
This compares with fewer than 19,000 arrivals in Italy and about 350 deaths in the first three months of 2016.
“We have yet to complete March, and we are already racing at a pace of arrivals that has exceeded anything we’ve seen before in the Mediterranean,” IOM spokesman Joel Millman said earlier this week.
“This is typical of spring, getting very busy, but it’s not typical to have the numbers be so high this early and the corresponding deaths that go with it.”