About 500 migrants need to be rescued from a boat off the coast of Sicily, the Alarm Phone charity said on Friday, amid a dramatic increase in sea arrivals to Italy and a recent deadly shipwreck.
The charity, which picks up calls from migrant vessels in distress, said on Twitter it had alerted "competent authorities" and urged them: "Don't waste time: send help immediately!".
The boat left from Libya and is "south of Sicily" in Italy's search and rescue (SAR) maritime zone, Alarm Phone added. The EU border force Frontex, which operates spotter flights off the Italian coast, declined immediate comment.
Italy's migrant sea rescue capabilities have come under scrutiny following a February 26 shipwreck near the southern region of Calabria, in which at least 72 migrants died.
Police vessels tried but failed to intercept the migrants' wooden boat due to adverse weather, and the coast guard, better equipped to face rough seas, was not immediately activated.
Italy's rightist government responded Thursday with tougher jail penalties for migrant smugglers and pledges to stop their illegal boat trips, while opening up legal migration channels.
The announcement has not stopped irregular migrant inflows, with almost 3,000 people reaching Italy since Wednesday, compared to around 1,300 for the whole of March last year.
As many as 1,869 migrants from 41 separate boats arrived on the island of Lampedusa alone on Thursday, the ANSA news agency said, calling it a record.