ROME (Reuters) - A migrant boat capsized in the Mediterranean on Thursday and while about 100 passengers were rescued, an unknown number are feared dead, officials said.

All told, the Italian coastguard said it had helped save 4,000 migrants today, in 22 operations. That brings the total number Italy has helped rescue since Monday to more than 10,000 - a sign that Europe’s immigration crisis is far from over.

Separately, Libya’s coastguard said it had stopped six boats carrying more than 750 migrants off the country’s western coast, and had recovered four bodies.

Thursday’s shipwreck was the second in two days as sea crossings accelerate amid good weather. Five people were confirmed to have died when a large fishing boat flipped over in the sea on Wednesday.

Europe’s worst immigration crisis since World War Two has led to more than 8,000 deaths in 2 1/2 years, the International Organization for Migration estimates.

Boat arrivals in Italy have risen sharply this week with warm weather and calm seas, and about 20 rescue operations are currently under way, a coastguard spokesman said.

Based on initial pictures from the aircraft that spotted the boat that overturned on Thursday, 20 to 30 people are feared to have died in the shipwreck, a spokesman for European Union’s Sophia naval mission said.

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Italy’s coastguard declined to estimate how many may have died. “We don’t know how many people were on board,” the coastguard spokesman said.

The non-profit organization Sea-Watch, which has a rescue ship in the area, said it was about to recover one body from the sea following a shipwreck and that it had picked up 115 migrants from a rubber boat.

“Aboard the boats are many Syrian and Iraqi people,” Sea-Watch said on Facebook. A European Union agreement to send migrants back to Turkey from Greece is forcing refugees to travel from North Africa to Europe, the group said.

So far, migrants from the Middle East have mostly traveled through Turkey to Greece. Most migrants who came to Italy by boat came from Africa.

Libya’s coastguard has blocked more than 2,000 migrants trying to leave Libyan waters for Europe this week.

On Thursday, it intercepted four boats early in the morning carrying about 550 people near the western port of Sabratha, Tripoli coastguard spokesman Ayoub Qassem said. Hours later, another 216 migrants were picked up from two boats off Zuwara, and the bodies of four migrants were found in the area, he said.

Through Tuesday, total sea arrivals in Italy had fallen by 9 percent this year to 37,743, according to the Interior Ministry. The country’s migrant shelters are under pressure to house 115,507 migrants, about twice as many as two years ago.

Some 650 migrants and the five dead bodies picked up by the Italian navy arrived at the Sicilian city of Porto Empedocle, where the migrants will get food and clothing. Officials will try to identify them before sending them to shelters.