The Catholic and Protestant leaders of Northern Ireland's coalition government jointly pledged to crush Irish Republican Army dissidents in an exceptional show of unity Tuesday after the third killing in two days claimed by an IRA splinter group.

The Continuity IRA said in a message to Belfast media that it carried out the fatal shooting of a policeman Monday night in a religiously divided town southwest of Belfast — 48 hours after the killing of two British soldiers claimed by the Real IRA. The killings appeared designed to undermine the unity government as its leaders prepared to leave for a high-profile U.S. tour capped by their first meeting with President Barack Obama at the White House on St. Patrick's Day, March 17.

The leaders, First Minister Peter Robinson and Deputy First Minister Martin McGuinness, postponed that trip for the second time and appeared shoulder to shoulder at a press conference alongside Northern Ireland's police commander, Chief Constable Hugh Orde.

McGuinness, a former IRA commander whose Sinn Fein party represents the Irish Catholic minority, decried the dissidents as "traitors to the island of Ireland."

He called for supporters to break their traditional code of silence and pass tips to the police.

"I want to join with Peter to wholeheartedly appeal to everyone, and anyone, who has any information whatsoever about these killings, to pass that information to the police, north and south," said McGuinness, who throughout the IRA's 1970-97 campaign supported the killing of police. Until two years ago he withheld public statements of support for law-enforcement officials.

"We need to pledge our support to Hugh Orde," McGuinness said as the Englishman stood beside him.

The Continuity IRA said in a message using a prearranged code word that it killed Constable Stephen Carroll, 48, as he sat in his patrol car Monday night in Craigavon. The breakaway group threatened to keep targeting police "as long as there is British involvement in Ireland."

The Real IRA fatally shot two soldiers and wounded four other people Saturday in an attack on a British army base west of Belfast.

The attacks were the first killings of British security forces in Northern Ireland since 1998 — the year that rival British Protestant and Irish Catholic politicians struck a peace deal designed to leave behind decades of bloodshed and promote a future based on cooperation and compromise.

For more than a decade IRA dissidents have been trying to mount attacks in hopes of reversing the results of ongoing political negotiations, which also delivered IRA disarmament in 2005, the rise of the Catholic-Protestant administration in May 2007 and the withdrawal of British troops from security duties two months later.

Analysts and anti-terrorist agencies say the Real IRA and Continuity IRA share identical aims and, despite their competitive rivalry, have cooperated in the past on planning and carrying out attacks.

But Orde said he believed they were operating independently at the moment — and could be motivated by a desire to outdo the other.

"I'm confident that we do not have some concerted effort by one group," he said.

Orde said his officers were on their guard for dissident ambushes before Monday night's attack, which looked like "a deliberate setup."

He said police had received a call from a terrified woman who reported that a street gang had shattered her window.

He said the officers "stood off for a sensible period of time" to check for any signs of a trap. Then two carloads of police drove in to the area.

Carroll, a 23-year veteran, was sitting in the car providing cover to the other unit when he was shot in the head through the car's rear window. A lone man was seen running away, police said.

Officers raided two homes Tuesday in a Catholic district that overlooks the spot where the policeman was killed. Forensic specialists seized documents, clothing and other materials but no arrests were reported.

Orde said the killing of the policeman exhibited "a different style" than Saturday's shooting outside the Massereene army base in Antrim, where the Real IRA opened fire on four off-duty, unarmed soldiers as they collected food from two Domino's Pizza couriers. All six suffered multiple gunshot wounds. The four survivors remained in serious but stable condition Tuesday.

In its claim of responsibility, the Real IRA said it shot the pizza men — a 19-year-old Antrim man and a 32-year-old Polish immigrant — because they were "collaborating" with the enemy.

The old IRA killed nearly 1,800 people, mostly soldiers and police before renouncing violence and disarming in 2005. It enjoyed strong support in the 1970s amid deep-seated Catholic grievances about economic disadvantage and discrimination. But decades of British reforms and effective Catholic campaigning have improved the fortunes of Northern Ireland's Irish nationalist minority and reduced support for IRA dissidents.

Belfast commentator Brian Feeney said Catholics' increasingly middle-class status and newfound political power mean "there are no nationalist grievances left to exploit."