Rome, Italy: The Italian coastguard said it had coordinated the rescue of about 4,200 migrants sailing across the Mediterranean Sea on Friday, but also found 17 corpses on several of the rickety boats.

Distress calls were made from 22 different boats, many off Libya but also off the southern Italian coast.

The total number of people rescued in 24 hours is one of the highest in recent years but the coastguard could not confirm if it was a record. So far, the busiest days this year have been the rescue of 3,791 migrants on 12 April and 3,690 on 2 May.

The 17 bodies were found on three inflatable dinghies, from which more than 300 other migrants were rescued alive, the Italian navy said on Twitter.

The navy’s press office, contacted by AFP, was not immediately able to say how the migrants died.

But the Italian authorities have in the past spoken of the harsh conditions faced by the migrants at sea, where they have to endure extreme weather changes and are at risk of hunger, thirst and violence on board the often crammed and flimsy vessels.

Friday’s rescue operations were led by the Italian coastguard and included the help of Italian, German and Irish naval ships working under the auspices of the EU’s Frontex border agency.

A similar international maritime rescue mission on Thursday saw more than 700 migrants helped to safety off the coast of Sicily after they had set sail from Libya in six boats.

Overall, more than 40,400 boat migrants—many of them fleeing conflict and poverty in countries such as Syria and Eritrea—have arrived in Italy since the start of the year, similar to the same period last year.

So far this year, some 1,770 migrants have perished on the hazardous journey to Europe, according to a latest International Organisation for Migration (IOM) report which does not include Friday’s rescue, a 30-fold increase on the same period in 2014.

The huge spike in the number of people trying to cross the Mediterranean Sea in recent weeks has been attributed to the worsening security situation in Libya—the staging post for most of the crossings—as well as milder weather.

‘It happens a lot in waves, you could have a few days where nothing happens, then there can be a high number of arrivals at the same time,’ Flavio di Giacomo, a spokesman for the IOM in Italy, told AFP.

After a string of deadly shipwrecks that sparked global alarm, EU ministers this month approved plans for a military operation to fight people smugglers in the Med, although proposals to destroy traffickers’ boats in Libyan waters still need UN approval.

The European Commission also unveiled plans to make the rest of the 28-nation EU share the burden of frontline states such as Italy, Greece and Malta when it comes to taking in migrants, although some countries such as Britain are opposed.

While massive, this year’s numbers are level with last year when authorities registered 41,243 arrivals between January and 31 May. A difference is however being felt in Greece which has registered 37,000 arrivals since the beginning of the year—already 3,000 more than in the whole of 2014, said Di Giacomo.