MILAN—The Italian navy and a number of search and rescue boats have saved nearly 7,000 migrants and refugees off the Libyan coast since Monday, in one of the largest ever rescue operations in the Mediterranean, an International Organization for Migration official said.

Flavio di Giacomo, spokesman for the IOM in Italy, said the number of those rescued was set to grow as rescue operations were still ongoing.

Calmer waters may have prompted such a high number of migrants to attempt the crossing simultaneously, Mr. Di Giacomo said, after days in which they likely waited in Libya because of choppy seas.

Until Monday, the migrants and refugees who reached Italy—most of whom had been rescued south of the island of Sicily—in August numbered around 12,000, or slightly more than half the arrivals in the same month last year. Since the beginning of the year, 113,000 migrants reached Italy, only slightly less than in the same period of last year.

However, the closure of European borders means migrants are bottled up in Italy, which is scrambling to find places to host them. Italian reception centers now shelter 145,000 migrants and refugees, an increase of more than 50% since the beginning of the year.

Migrants camping in border towns of Ventimiglia and Como hoping to reach France or Switzerland have grabbed headlines in recent weeks due to tensions with police and residents.

Alberto Sinigallia, chairman of Fondazione Progetto Arca, an aid group working with migrants and refugees in the Milan area, said that, up to last year, migrants were stopping typically four days at the reception centers managed by Arca. Now, they have started to stop indefinitely, while they wait for their asylum requests to be processed.

"We are very worried the number may suddenly surge," Mr. Sinigallia said. "If it does, we will not be able to cope."

Since Monday, migrants have been rescued from 44 rubber dinghies, eight small wooden boats and two large vessels, Mr. Di Giacomo said. One of the large boats was carrying around 700 migrants.

Doctors Without Borders, an aid group which participated to the operations with its boat Dignity 1, saved 435 people, or 35 more than its standard capacity "due to the extreme situation," the organization said in a statement.

Its medical teams treated people who were exhausted and experiencing bloody diarrhea, dehydration, fever, hypothermia, and skin diseases. It also rescued two premature babies delivered at 8 months who were 5 days old, whose mother was traveling alone with them.

More than 3,000 people lost their lives this year in the Mediterranean, making the area one of the deadliest migrants' routes in the world.

Write to Giovanni Legorano at giovanni.legorano@wsj.com

(END) Dow Jones Newswires

August 30, 2016 08:45 ET (12:45 GMT)