A fishing boat carrying an estimated 600 migrants capsized on Wednesday in the Mediterranean north of Libya, and at least 150 people were spotted in the sea, Irish military officials said.

The Irish naval vessel Le Niamh was one of several ships requested by the Italian coast guard to speed to the rescue of the overturned boat shortly before noon, Irish Capt. Donal Gallagher told The Associated Press by phone.

Gallagher said that according to preliminary reports some 150 migrants were spotted in the water after the smugglers' boat, which was estimated to have been carrying 600 migrants, overturned. "An Italian (military) helicopter has dropped additional life rafts" into the sea, Gallagher said.

Also involved in the rescue were an Italian vessel and a boat operated by Doctors Without Borders. Non-governmental organizations often join in migrant sea rescue operations, which are coordinated by Italy's coast guard and are now under the umbrella of a European Union task force known as Triton. The distressed vessel was reported to be 110 kilometers (about 75 miles) northwest of Tripoli, Libya's capital.

Officials have yet to determine what caused the capsizing.

The International Organization for Migration, a human rights group, has said nearly 2,000 migrants are believed to have lost their lives at sea since the start of this year, but the exact toll of dead is not known. In April, a smuggling boat crammed with an estimated 800 migrants overturned, also off Libya's coast where smugglers operate. Only 28 survivors, including two alleged smugglers, were found in that tragedy.

Fleeing war, persecution and poverty, the migrants travel overland for weeks or months from sub-Saharan Africa, the Horn of Africa, the Middle East and parts of Asia to reach Libya. There they set sail in flimsy motorized rubber dinghies or rickety old fishing boats. When the boats have problems, someone aboard contacts the coast guard by satellite phone requesting rescue. Other boats in distress are spotted by Triton air surveillance.

Most of the migrants hope to find asylum, relatives or jobs, mainly in northern Europe.