More than 6,000 migrants have been rescued on the central Mediterranean route from Libya to Italy in the last few days, as greater numbers take to the sea in warmer weather, the International Organization for Migration (IOM) said on Tuesday.

Some 500 migrants are believed to have drown so far this year, including 22 deaths just reported by the Libyan coast guard, as more than 20,000 people arrived in Europe, mainly to Italy, spokesman Joel Millman told a news briefing.

"We have yet to complete March, and we are already racing at a pace of arrivals that has exceeded anything we've seen before in the Mediterranean. This is typical of spring, getting very busy," he said. "But it's not typical to have the numbers be so high this early and the corresponding deaths that go with it."