David Cameron "bears some responsibility" for the stream of refugees fleeing to Europe from Libya, Ed Miliband insisted as he dismissed Tory claims that he was playing politics with the tragedy in the Mediterranean.
The Labour leader said it was "nonsense" that he was seeking to link British foreign policy to the deaths of hundreds in capsized boats after facing Tory claims of a "provocative and shameful intervention".
Cameron, back on the campaign trail after pledging UK assistance to rescue operations at an emergency EU summit, suggested the "ill-judged comments" raised doubts about Miliband's suitability for office.
He said during a campaign visit to highlight the Conservatives' "English manifesto": "I have learned as prime minister that it is so important in a dangerous and uncertain world that you show clarity, consistency and strength on these foreign policy issues.People will look at these ill-judged remarks and they will reach their own conclusions."
In the latest in a wave of disasters, a boat carrying migrants hoping to reach Italy capsized off the coast of Libya, killing an estimated 800 migrants on board.
Following a speech on foreign policy at the Chatham House think-tank this morning, Miliband said the "only people trying to whip up a big storm about this are the Conservative Party". Asked if he was suggesting Cameron had "blood on his hands", the Labour leader said that was "wide of the mark".
Migrants being rescued from the Aegean sea, after three people have died when a boat carrying tens of migrants ran aground
A coffin of a migrant who drowned when a fishing boat off the Libyan coast
In his speech, Miliaband said Cameron was "wrong" to suppose that the country could emerge from the ousting of Muammar Gaddafi without more outside help - claiming its descent into chaos "should have been anticipated and could have been avoided".
"Britain could have played its part in ensuring the international community stood by the people of Libya in practice, rather than standing behind the unfounded hopes of potential hopes only in principle."
Ed Miliaband on Labour's General Election campaign
Two polls today gave Conservatives a four-point lead over Labour - the kind of advantage which could leave Cameron with 20 to 25 more MPs than Miliband but short of a majority in the House of Commons.
One survey, for the Daily Mirror, showed Labour dipping below 30% for the first time in the general election campaign. But another, in The Sun, had Labour in a two-point lead on 35%, to the Tories 33%.
In his speech which aims to claim traditional Conservative territory in the General Election battle, Miliband also claimed that Labour is now the party best placed to maintain the security of the UK, because of Tory plans for cuts which would "undermine" the armed forces.
Citing figures from the independent Institute for Fiscal Studies suggesting that government departments including the Ministry of Defence would face deep cuts as part of Conservative deficit-reduction plans, Miliband said the Tories are set on "an ideological project to cut the size of the state" which would harm the security of the country.
"I want to be absolutely clear that amongst the reasons we reject the extreme spending cuts that the Conservative Party propose is that they would be truly catastrophic for the future of our armed forces," he said.
"The IFS set out yesterday that they would mean at least 18% budget reductions for departments like the Ministry of Defence - significantly more than the cuts it has had over the course of the last Parliament. Conservative assurances to protect specific parts of the defence budget are meaningless in that world; they simply will not be delivered, they will be broken promises.
"Labour simply will not take the extreme approach our opponents propose. I am not going to sacrifice the defence and security of our country on an ideological commitment to a significantly smaller state.
"Indeed, we are now in an unprecedented situation going into this election. It is now the Labour Party which is much better positioned to find the resources in the next parliament that our armed forces need to maintain the security of the United Kingdom and play our part in maintaining the security of the world."
In the wide-ranging speech, Miliband attempted to put some of his own party's ghosts to rest by insisting that he has learned the lessons of the Iraq war.
Miliband set out the terms for military action under a Labour government and highlight the threats Britain is facing internationally, including from Islamic State, also known as Isil.
In stark contrast to the Conservative promise of an in/out referendum on Britain's membership of the European Union, Miliband said he "will never put our national interest at risk by threatening to leave".
Miliband also said that Britain has "more authority when we work" with global allies and will pledge to strengthen the UK's position in international institutions such as the United Nations, Nato, the Commonwealth and the European Union.
"It is time to reject the small-minded isolationism that has characterised this government, diminished the office of prime minister and shrunk the influence of Britain," he said
"Because this government's approach has and weakened Britain at a time when the challenges are perhaps greater and more complex than at any time since the Second World War."