Northern Ireland's deputy First Minister Martin McGuinness has said supporters of dissident republicans need to "get real" and realise they are a "ragbag" of criminals out for their own ends.
Speaking after the murder of prison officer David Black yesterday, Mr McGuiness said they swim "in a sea of criminality and drugs" while wrapping themselves in a flag of political convenience.
"People need to get real. They need to recognise the danger that a tiny number of people can represent to human beings and they need to recognise that the world has changed, that over the course of the last 15 years we have built something, all of us together, which we can be very proud of."
Earlier, the Sinn Fein veteran and First Minister Peter Robinson stood shoulder to shoulder with Irish prime minister Taoiseach Enda Kenny and his deputy Tßnaiste Eamon Gilmore after a meeting in Armagh as they jointly denounced the killing.
Asked about the motive behind the attack of prison officer David Black, Northern Ireland Secretary Theresa Villiers told the House of Commons:
It is too early to be able to say with any certainty exactly why David Black was targeted. But as (police) made clear yesterday, this attack does have all the hallmarks of dissident republicanism.
It is also too early to say whether there is any connection with the prison dispute that is currently taking place in Northern Ireland. Whatever the dispute about prison conditions, nothing could possibly justify what has taken place.
The US ambassador to Britain, Louis Susman, has been briefed on the killing of prison officer David Black in Northern Ireland, Theresa Villiers has said.
The Northern Ireland Secretary said Mr Susman added his condemnation of the atrocity.
Mrs Villiers also said US secretary of state Hillary Clinton, whose husband Bill played a key role in the 1998 Good Friday Agreement peace deal when he was president, "retains a close and strong interest in Northern Ireland".
She added: "I'm sure she shares the concern that has been expressed in the House today."
In an urgent statement to MPs, Northern Ireland Secretary Theresa Villiers sent her "deepest condolences" to David Black's widow Yvonne, son Kyle and daughter Kyra for their "devastating loss" and vowed to catch his killers.
"The Government will do whatever we can to help the PSNI bring the perpetrators of this atrocity to justice," she said. "We will continue to do everything in our power to ensure the terrorists do not succeed."
Mrs Villiers said yesterday's killing showed the threat posed by dissident republicans.
"The numbers involved in terrorism activities are small, but these groupings have the capability and they have lethal intent," she said. "They can still ruin lives despite their lack of support in the community, so we remain vigilant."