MILAN — The unabated flow of migrants fleeing instability in Libya brought a new horror on Friday: The discovery of 20 migrants adrift at sea who had suffered grave burn injuries in a cooking gas explosion before departing Libya, and then were forced onto a smuggler’s boat without treatment.

Italian ships have picked up 10,000 people, many of them refugees of war and persecution, over the past week, an unprecedented number in such a short period. The influx is putting pressure on Italy’s shelter system and raising calls for a better response to the emergency.

Friday’s rescue comes after the feared drowning of more than 400 migrants in two shipwrecks in the last week, bringing to more than 900 the number of people who have died or gone missing so far this year making the perilous crossing.

In Washington, President Obama pledged more intense cooperation with Italy on threats coming from the instability in Libya, which has contributed to the influx of migrants across the Mediterranean. Libya, the closest point in north Africa to Italy, is a transit point for migrants.