Nearly 100 people are missing after a boat packed with refugees and migrants sank off the Libyan coast, killing at least eight, according to the Italian coastguard.

Search operations continued after nightfall in poor weather and sea conditions, officials said on Saturday.

"The bodies of eight people have been recovered. Four people have been saved, and they say 107 migrants were onboard the boat," the coastguard's press office said.

A French warship, patrolling under the European Union's Frontex border operation, picked up the survivors and two merchant ships were heading for the area, located about 50km north of the Libyan coast.

A Frontex jet and an Italian naval helicopter also joined the search and rescue operation.

On Friday, around 550 refugees and migrants were picked up from four inflatable dinghies by Italian coastguard vessels, an Italian naval ship, an NGO boat and a merchant vessel.

In recent years, Italy has been on the front line of refugees and migrants arriving across the Mediterranean Sea and has been pushing for agreements with governments in North Africa to facilitate returns.

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People smugglers have exploited the chaos in Libya since the 2011 ousting of leader Muammar Gaddafi to traffic refugees and migrants in boats to Italy 300km away.

According to the Italian interior ministry, more than 180,000 refugees and migrants landed in Italy last year, an annual record.

The United Nations has said more than 5,000 people died in 2016 trying to cross the Mediterranean, most of them on the Libya-Italy route.

In another incident, the bodies of six people were found washed up on beaches near Algeciras, the port on the southern tip of Spain near Gibraltar, the Spanish sea rescue service said on Saturday.

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The six were all men, apparently from sub-Saharan Africa.

More than 361,000 refugees and migrants reached European shores by boat last year, according to the UN's refugee agency.

In 2015, more than a million people fleeing war and economic devastation made the same journey.