Britain is a country that "doesn't walk on by", David Cameron said as HMS Bulwark undertook another rescue mission off Libya.
The Royal Navy warship is picking up at least 500 migrants found in four boats in the seas off the north African country.
Arriving at the G7 summit in Garmisch-Partenkirchen, Germany, the Prime Minister said the flagship had been deployed because the UK is a "country with a conscience".
But he warned that the causes of the mass exodus from Libya must be dealt with, not just the consequences.
Mr Cameron said: "HMS Bulwark, the Royal Navy flagship, is in the Mediterranean because we want to save lives.
"Britain is a country that doesn't walk on by. We are a country with a conscience, and that's right.
"But we also need to do more to stop these people leaving their countries in the first place. That's what we are using our aid budget for, that's why we will be talking here at the G7 about how we try and put a Libyan government together.
"We need to deal with the causes of this migration, not simply with its consequences."