About seven migrants from sub-Saharan Africa have been found dead in an abandoned refrigerator truck near the Libyan capital, a senior official in the fight against illegal immigration has confirmed to AFP.

“There were 35 illegal migrants on board, but seven of them had already died, and the 28 survivors have received medical treatment and are in good health,” Adel Mostafa said.

He added that the dead migrants, who were found on Sunday, had almost certainly asphyxiated.

The official said he did not know what had prompted the smuggler to abandon the vehicle and the people inside it.

Many migrants from sub-Saharan Africa are driven through southern Libya in trucks to the northern coast where they later attempt the perilous Mediterranean crossing to Italy in Europe some 300 kilometres (180 miles) away.

People traffickers have exploited the chaos in Libya since the 2011 uprising deposed and killed strongman Moamer Kadhafi to boost their lucrative but deadly trade.

According to International Organization for Migration and UNHCR figures, 1481 migrants have died and 1720 are missing since January 1.

In 2016, 181 000 migrants reached Europe via Italy, 90 percent of them from Libya.

By mid-April this year, Italy had registered nearly 42 500 migrants coming by sea, 97%  of them arriving from Libya.