MILAN (AP) — Italian officials say good weather is the main factor behind a spike in migrant crossings from Libya, with ships picking up 3,100 migrants over the past three days.

Some 712 migrants were picked up Thursday on top of 2,400 the two previous days, all grouped in small, flimsy rubber dinghies in international waters near Libya. Three bodies were found during Wednesday's rescues by mostly Italian ships.

"In these days there has been a window of weather that has been good for transiting this section of sea," Coast Guard Cmdr. Filippo Marini said.

The U.N. refugee agency says 9,800 migrants have arrived in Italy this year through Wednesday, compared with 7,400 in the same period last year. Spokeswoman Barbara Molinario said that it was too early to speak of trends and this kind of concentration in short periods is not unprecedented — even if periods of calm seas in the winter are limited.

Most of the arrivals this year are migrants from sub-Saharan Africa, Molinario said, a flow that indicates that the closures along the route from Greece have not led to a shift of the people in that area toward Libya.

"They are the same that transited Libya last year, and not the Syrians, Afghan and Iraqis who go through Greece," Molinario said. "There is no sign of a change of route, although we don't exclude that there can be one."

The International Organization for Migration says other factors beyond weather may be at play. Spokesman Flavio di Giacomo said periodic upticks can be due to "strategy of the traffickers that we cannot know," such as the availability of boats in Libya.

Di Giacomo said that the concentration of successful rescues without shipwrecks indicates that rescue efforts in the Mediterranean are "very efficient."

"These are dinghies of terrible quality. It is impossible to think that they would be capable of reaching Lampedusa or Sicily," di Giacomo said. "The rescues are always in international waters, very close to Libya."