A boat packed with mainly African migrants bound for Italy has sunk off the Libyan coast with up to 200 people feared dead, officials say.

A security official in the western town of Zuwara, from where the overcrowded boat set off, said there were about 400 people on board.

Many appeared to have been trapped in the hold when it capsized.

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By late in the evening, the Libyan coast guard rescued about 200, of which 147 were brought to a detention facility for illegal migrants in Sabratha, west of Tripoli, the official said, asking not to be named.

Another local official and a journalist based in Zuwara confirmed the sinking but also had no information on casualties.

The migrants on board had been from sub-Saharan Africa, Pakistan, Syria, Morocco and Bangladesh, the security official said.

The Italian coast guard, which has been coordinating rescue operations with the European Union off the Libyan coast, could not immediately confirm a sinking.

The Rome-based authority later said some 1,430 people were rescued in numerous operations near Libya throughout the day on Thursday, and a merchant ship sent to the aid of a wooden boat carrying 125 people recovered two bodies.

Libya's coast guard has very limited capabilities, relying on small inflatables, tug boats and fishing vessels.

"We are working with very limited resources," a Libyan coast guard official said.

"Most of the boats we use are fishing boats that we borrow from their owners."

Reports said another boat with about 50 people on board had also capsized on Thursday.

Smugglers exploit Libya's lawlessness

Zuwara, Libya's most western town located near the Tunisian border, is a major launch pad for smugglers shipping migrants to Italy.

Libya has turned into a transit route for migrants fleeing conflict and poverty to make it to Europe.

Cross-border smuggler networks exploit the country's lawlessness and chaos to bring Syrians into Libya via Egypt or nationals of sub-Saharan countries via Niger, Sudan and Chad.

More than 2,300 people have died this year in attempts to reach Europe by boat, compared with 3,279 during the whole of last year, according to the International Organisation for Migration.

As many as 70 refugees were found dead in a parked truck in Austria near the Hungarian border on Thursday, and German chancellor Angela Merkel said the discovery had shaken European leaders discussing the migrant crisis at a Balkans summit.

On Wednesday, at least 55 bodies were discovered on three overcrowded migrant boats, 52 of whom were found in the hold of one wooden vessel off Libya's coast.

Libya has been struggling to cope with an influx of migrants, putting them in overcrowded makeshift detention facilities such as schools or military barracks where they live in poor conditions lacking medical care.

The North African country used to deport migrants it caught but with fighting between armed groups having cut off land border crossings to Niger, Algeria and Chad many stay months or years in detention facilities.

Reuters/AFP