The number of migrants killed while trying to reach Europe since the start of 2019 has risen to 900, the UN said, after 40 drowned off the Libyan coast when their boat capsized.
A migrant support group called Alarm Phone received a call from those on board the sinking ship.
It said the migrants “were in severe distress, crying and shouting, telling us people had died already”.
The Libyan coastguard said it rescued 65 migrants from the wreckage. Most were said to be of Sudanese origin.
The search for any survivors ended on Tuesday, AP reported.
Ayoub Gassim, a spokesman for the Libyan coastguard, said among the dead were a woman and child from Morocco, whose bodies were found near the west Libyan city of Khoms.
The UN's International Organisation for Migration says 859 migrants have died in the Mediterranean from the start of this year to August 22.
Aid organisations operating between Europe and Africa have complained that a continuing standoff with European authorities has endangered the lives of migrants.
Axel Steier, the co-founder of the German NGO Mission Lifeline, told The National his ship Eleonore had been prevented from rescuing migrants elsewhere in the Mediterranean because it was stuck off Malta without permission to dock.
“The problem with the politics at the moment is there are thousands of migrants coming from Libya every year," Mr Steier said.
"The problem is not the number, it is that the politics is not moving forward.
“There is no interest in the states to help in this case and now our ship is bound we cannot go out again and rescue more people. They have to wait."
He said rough seas could endanger the lives of the migrants and crew on board the Eleonore, making it unclear how long the standoff would last.
"We don't know how long. It depends on the weather, it depends on the mental situation of the people," said Mr Steier.
“If we get bad weather we will have to send a mayday. Water can come in. If we see life is in danger then there is no room for discussion any more."
Malta has repeatedly rebuffed rescue vessels in the Mediterranean on the grounds that it does not have the capacity to process the new arrivals.
Italy, which is on the front line of the European migrant crisis, has introduced fines against migrant rescue charities in the Mediterranean.
At the start of August, the Italian Parliament introduced fines of up to €1 million (Dh4.1m) against the vessels carrying migrants into its waters, saying it would arrest their captains for doing so.
Former Italian interior minister Matteo Salvini, the hard-line anti-migrant politician, signed a decree on Tuesday evening, along with ministers Danilo Toninelli and Elisabetta Trenta.
The decree banned the Eleonore vessel from entering Italy.
On Tuesday, a Spanish warship collected 15 rescued migrants from an Italian port having sailed halfway across the Mediterranean. The gesture sharply contrasted with the refusal of Italy to accept the migrants.
Madrid sent the ship a week ago at the end of a long standoff between Italian authorities and a Spanish-registered private rescue boat that plucked more than 100 people, most of them Africans, from seas off Libya.
The Libyan coastguard said it intercepted hundreds of migrants trying to reach Europe in August.
The country has become a centre for mostly Sub-Saharan African migrants who have crossed its porous borders since the 2011 uprising that overthrew dictator Muammar Qaddafi.