David Cameron announced that he would break off from election campaigning to attend a meeting of the European Council in Brussels.
The news came as Italian and Maltese ships responded to distress calls from two boats with hundreds of people on board. In another incident, at least three people are believed to have died after a boat carrying migrants ran aground off the coast of the Greek island of Rhodes.
The latest reports came as rescue teams continued searching for bodies and survivors after up to 900 people were feared to have drowned off the coast of Libya at the weekend in the worst disaster of the emergency so far. Shocking details of the incident – described by Mr Cameron as “a dark day for Europe” – have begun to emerge.
Many children are said to have been on board and there are claims that hundreds of migrants were locked in a hold by people smugglers.
It is thought the 20-metre vessel capsized after migrants rushed to one side of the craft when they saw a Portuguese merchant ship approaching their craft.
Gemma Parkin, of Save the Children, said 28 survivors of the shipwreck are being brought to Sicily.
Speaking from Catania, on the east coast of the island, she said no women or children are believed to have survived.
“There is a huge number of people presumed dead now,” she said.
Calling for an emergency summit, she added: “Whatever you think of migration, you can’t let children drown in this way.”
Earlier, the Prime Minister blamed trafficking gangs as pressure mounted to resume full-scale search and rescue missions.
He said: “It is a very dark day for Europe, it really is horrific the scenes that we have all witnessed on our television screens, the loss of life. We should put the blame squarely with the criminal human traffickers who are the ones managing, promoting and selling this trade, this trade in human life. “
He added: “We are doing everything we can to try and stop them.”
The rising death toll comes as thousands of desperate migrants attempt to cross from North Africa to Europe, often crammed into unsafe boats.
Up to 1,500 have drowned this year alone and the Italian Navy and coastguard brought more than 13,000 people ashore in the space of just a week.
An Italian scheme called Mare Nostrum was abandoned late last year despite rescuing tens of thousands of people making the treacherous journey and was replaced with a more limited EU border security operation.
The UK had argued that search and rescue operations might encourage more illegal immigrants.