Jeremy Corbyn has refused to single out the IRA for condemnation when pressed over his past campaigning activities.
The Labour leader said “all bombing is wrong”, as he was repeatedly asked to condemn the IRA alone for its role in the Troubles.
He said he had worked hard to help secure peace in Northern Ireland.
But Democratic Unionist Party leader Arlene Foster launched an attack on the Labour leader, saying he was “beyond the political pale”.
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Mr Corbyn’s campaigning during the Troubles has come under renewed scrutiny since his election as Labour Party leader.
He and shadow chancellor John McDonnell have been prominent supporters of Sinn Féin; before the IRA ceasefire they controversially met the party on a number of occasions in Westminster during the 1990s.
The Labour leader has said the meetings formed part of his attempt to bring about peace in Northern Ireland at that time.
In an exchange with interviewer Sophy Ridge on Sky News on Sunday, he was asked to condemn the IRA. He replied that he “condemned all those that do bombing, all those on both sides”.
Asked again, he added: “There were Loyalist bombs as well. I condemn all the bombing by both the Loyalists and the IRA.”
He added: “I recognised that you had to bring about a peace process in Ireland. I did my best to assist in that process and that is the way you bring about peace anywhere in the world”.
Speaking on the BBC’s Sunday Politics, shadow chief secretary to the Treasury Peter Dowd said the comments had to be seen “in the context of Jeremy Corbyn for many years trying to move the peace process along”.
But the Conservatives said people would be “outraged” that the Labour leader would not “unequivocally condemn the IRA”.
Ms Foster, Northern Ireland’s former first minister, is expected to attack the Labour leader in a speech in London on Monday.
“Political leaders from Northern Ireland understandably often remain neutral, at least publicly, about the outcome of a Westminster general election,” she will say.
“But this election is different. While Theresa May is well within the political mainstream and has proven herself to be a solid and reliable unionist, Jeremy Corbyn is beyond the political pale.
“It is hard to take seriously the democratic credentials of a man who was so close to the political representatives of the IRA at the height of the Troubles.”