GENEVA -  About 90 people are feared drowned after a smugglers' boat carrying mostly Pakistani migrants capsized in the Mediterranean Sea off Libya's coast early on Friday, the UN's migration agency said.

Survivors told aid workers that most of the migrants on board were Pakistanis, who make up a growing number of those attempting the hazardous voyage across the Mediterranean to Italy from North Africa.

The tragedy happened off the coast of Zuwara in the early hours Friday, International Organisation for Migration (IOM) spokeswoman Olivia Headon told reporters in Geneva by phone from Tunis.

"At least 90 migrants are reported to have drowned, when a boat capsized off the coast of Libya this morning," the IOM said in a statement. The agency said that "10 bodies are reported to have washed up on Libyan shores", including those of two Libyans and eight Pakistanis.

Two survivors from the disaster had swum to shore, while another was rescued by a fishing boat, it added.

The agency has repeatedly issued warnings over the extreme dangers facing migrants who try to reach Europe via the so-called central Mediterranean route, which connects Libya to Italy.

IOM said Friday that more than 6,600 migrants and refugees had already entered Europe by sea this year, with central Mediterranean route crossings to Italy accounting for nearly 65 percent of the entries.

It voiced surprise that Libyans were among the dead, pointing out that only 29 Libyan nationals were rescued or intercepted trying to cross the Mediterranean in all of 2017, with no Libyan deaths recorded last year.

Asked if it was common to see Libyans among the migrants trying to cross to Europe, IOM spokesman Joel Millman said "we haven't (really) seen that before."

"They could have been smugglers," he told AFP.

The large number of Pakistanis found dead could meanwhile hint at a shift in migration trends.

IOM pointed out that Pakistanis made up the 13th largest group trying to cross the Mediterranean to Europe last year, with 3,138 of them arriving in Italy in 2017, and no recorded sea deaths. But they have already climbed to third place this year, with an estimated 240 Pakistanis reaching Italy in January, compared to just nine during the same month last year.

Drownings in the Mediterranean began surging in 2013 as Europe's worst migration crisis since World War II began picking up speed, with hundreds of thousands of people fleeing war and poverty in the Middle East and elsewhere.

Over the past five years, more than 16,000 people have died trying to make the perilous crossing to Europe, according to IOM numbers.

Excluding Friday's tragedy, 246 migrants and refugees have already died trying to cross the Mediterranean since the beginning of the year, compared to 254 casualties during the first month of 2017.

Two hundred and eighteen of the deaths this year occurred on the central route, IOM said, while 28 happened on the western route that links North Africa to Spain.

No deaths have been recorded this year on the eastern Mediterranean route that connects Turkey and Greece, used by 1,089 migrants so far in 2018.

The EU last year reached controversial agreements with chaos-wracked Libya to stem the flow of migrants from that country, following a more comprehensive deal with Turkey in 2016, which sharply reduced the numbers crossing to Greece. Casualties in the eastern Mediterranean have dropped dramatically since then.

In the 22 months since the deal with Turkey was reached in April 2016, fatalities on that route have fallen to an average of 6.75 per month, from 96.25 per month during the year prior to the agreement, IOM said.

Another IOM spokesman, Leonard Doyle, told Reuters Television that the boat was believed to have left shore on Thursday before capsizing early on Friday morning. The tragedy demonstrates the continued allure of Europe for desperate migrants fleeing conflicts and poverty, Doyle said, despite tighter surveillance of the main smuggling routes by the Libyan coastguard, backed up by European cash and know-how.

“They (the migrants) are lured there by social media. They get onto a phone, they are promised El Dorado, they think life is going to be great. And before they know it, they are getting into the hands of awful criminal, extorting people – smugglers, traffickers, this dreadful, shocking torture,” he said.