Jeremy Corbyn has come under fire for saying it was a "tragedy" that Osama bin Laden was killed by the United States rather than being put on trial.
The Labour leadership frontrunner made the remarks shortly after the 2012 special forces raid on the al Qaida chief's Pakistan compound in which he and four others were shot dead.
In an interview for Iranian television, he suggested the assassination of the September 11 attacks' mastermind would result in deeper unrest.
It is the latest in a series of past comments and associations that the veteran left-winger has been forced to defend since emerging as the surprise favourite to succeed Ed Miliband.
In a clip from the Press TV show The Agenda, Mr Corbyn is heard complaining that there had been "no attempt whatsoever that I can see to arrest him and put him on trial, to go through that process". He went on: "This was an assassination attempt, and is yet another tragedy, upon a tragedy, upon a tragedy.
"The World Trade Center was a tragedy, the attack on Afghanistan was a tragedy, the war in Iraq was a tragedy. Tens of thousands of people have died. Torture has come back on to the world stage, been canonised virtually into law by Guantanamo and Bagram.
"Can’t we learn some lessons from this? Are we just going to sink deeper and deeper?
"The next stage will be an attempted assassination on Gaddafi and so it will go on. This will just make the world more dangerous and worse and worse and worse."
A spokesman for Mr Corbyn said he was "a total opponent of al Qaida, all it stands for".
But Liberal Democrat leader Tim Farron said Mr Corbyn was "utterly wrong".
"Bin Laden's death was not a tragedy. The tragedy was the 2,977 who died during that awful day. We remember them," he said.
It came as George Osborne claimed a Labour Party led by Jeremy Corbyn would pose a threat to national security by threatening the future of the UK's nuclear deterrent.
The Chancellor said "an unholy alliance of Labour's left-wing insurgents and the Scottish nationalists" would shatter decades of near-unbroken Westminster consensus in favour of maintaining a capability.
Both Mr Corbyn and the SNP are opposed to the renewal of the Trident missile system being pursued by the Conservative government.
But Mr Osborne, who is visiting the Faslane naval base, home of the nuclear submarine fleet, to announce a £500 million investment, said that would be "disastrous".
Amid suggestions that Conservatives were delighted at Mr Corbyn's surprise emergence as the favourite to lead the party, Mr Osborne insisted the contest must not be seen as a "a bit of a joke".
"On the contrary, I think we should take it deadly seriously," he wrote in The Sun.
"For the new unilateralists of British politics are a threat to our future national security and to our economic security. We're going to take on their dangerous arguments and defeat them."
The only breakdown in agreement over the need for a nuclear deterrent had being during the 1980s when Labour was dominated by the left, he said.
"Now that consensus, which is so important for our security and reliability as an ally, risks being shattered again by an unholy alliance of Labour's left-wing insurgents and the Scottish nationalists.
"This isn't an argument about the past - the return of the unilateralists to British politics threatens our nation's future security."
Despite Mr Corbyn's dominance of the campaign, Andy Burnham claimed enough Labour backers are still to make up their minds to swing the result of the party leadership race in his favour.
The bulk of the 550,000-plus available votes are thought already to have been cast but Mr Burnham will tell a rally on he is confident many of the undecided are switching back to his cause.
The other candidates are fellow shadow cabinet minister Yvette Cooper and Liz Kendall.
"Tens of thousands of people are still to vote and there is everything to play for, he will say - though his camp said they did not have any details of the number of votes cast.