The UN refugee agency said more than 700 refugees may have drowned in three Mediterranean Sea shipwrecks south of Italy in the past few days.

Speaking to Al Jazeera from Geneva, William Spindler, a UNHCR spokesman for Europe, said there had been a spate of incidents in the Mediterranean last week.

"The first one was on Wednesday, which was widely reported. A boat capsized, carrying about 600 people. Some of those people are still missing," Spindler said.

"Then there was a second incident on Thursday and a third one on Friday. In total, we fear that in these three incidents more than 700 people are missing, presumed drowned.

"There is a number of boats at the moment trying to make their way from Libya to Italy and several rescue operations are ongoing."

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Spindler said more needed to be done to stop refugees from taking to the sea in small boats.

"It is very difficult to prevent people from doing this. In our view, what needs to be done is to offer legal alternatives to the most vulnerable refugees to travel to Europe. The reason why people are undertaking these dangerous journeys is because they have no choice," he said.

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Carlotta Sami, a spokeswoman for UNHCR, told the Associated Press news agency by phone that an estimated 100 people were missing from a smugglers' boat that capsized on Wednesday.

She said about 550 others were missing from the boat that sank on Thursday.

Refugees said that boat, which was carrying about 670 people, did not have an engine and was being towed by another smuggling boat before it capsized.

About 25 people survived the incident, 79 others were rescued by patrol boats, and 15 bodies were recovered.

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Sami said 45 more bodies were recovered from the shipwreck on Friday and many more were reported missing.

In a separate development, 19 refugees were rescued from a boat in the English Channel, the AFP news agency reported, citing coastguard officials.