One official said the incidents took place over the past two days and search operations were continuing.

Gemini Pandya, a spokeswoman for the International Organisation for Migration (IOM), told Al Jazeera on Tuesday that the estimates of people feared drowned vary between 300 and 500.

She said high winds and possible over-crowding on the boats may have contributed to the incidents.

Libyan officials said it was not yet clear whether the two confirmed capsized boats and the two others spotted drifting off Libya all sailed from Libya, but the country is a popular departure point for African migrants trying to reach Europe.

Joint sea patrols

The deaths come as Italy and Libya are set to launch joint sea patrols in May in an effort aimed at stopping the influx of migrants trying to reach European countries.

Roberto Maroni, the Italian interior minister, said on Monday that the patrols would begin on May 15, and he expected that day to "mark the end" of illegal migration from North Africa.

"We have done everything possible to prevent arrivals," he said.

"We have signed a deal with Libya and the Libyan government committed to launch on May 15 patrols along the coasts with six vessels.

"Until then, we will continue to ask the Libyan authorities to intensify controls, but I guess that there will still be arrivals," Maroni said.

Nearly 37,000 immigrants deemed illegal landed on Italian coasts last year, a 75 per cent rise from 2007, according to the Italian interior ministry.

Rights groups say most migrants risk their lives in treacherous waters trying to reach Europe hoping to escape poverty in their home countries.

Pandya, of the IOM, said work continues to try to discourage people from migrating using smuggling networks. 

"There are lots of pressures that force people to take these incredibly dangerous risks. There are fewer and fewer legal means of migrating."