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."
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Last week Bulwark plucked 747 people from dangerously overcrowded boats in a similar operation and it is thought today's rescue will take the number of lives saved to more than 2,000 since it began its work there last month.
All European naval ships in the region were ordered to go to the area after intelligence suggested up to 14 migrant vessels were on the open water.
A Ministry of Defence spokeswoman said: "From first light, a Merlin helicopter from 814 Squadron on board HMS Bulwark has been conducting surveillance operations and has identified four migrant vessels in distress, containing a total of at least 500 people.
"HMS Bulwark has now commenced rescue operations to recover those in the vessels."
The 19,000-tonne assault ship was sent by the Government on May 4 to help search and rescue efforts in the Mediterranean in a bid to tackle a dramatic increase in the number of people dying trying to cross the sea.
It is estimated that more than 1,600 people have drowned so far this year trying to make the crossing.
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