The Taoiseach, Mr Ahern, has called for a crackdown on loyalist paramilitaries following the killing of a Catholic postal worker in north Belfast on Saturday.
Mr Daniel McColgan (20) was gunned down as he arrived for work at a sorting depot on the Rathcoole estate shortly before 5 a.m. The Red Hand Defenders, a cover name used by the Ulster Defence Association, admitted responsibility for his death.
Condemning the killing, Mr Ahern said he was concerned about the rapidly deteriorating situation in north Belfast. While official police figures showed that two-thirds of shootings and other violent incidents occurring there were carried out by loyalist paramilitaries, there had been few arrests.
"It would be surprising if some of the leaders of the loyalist gangs are not known and, of course, people have felt that something was going to happen because tension has built up over the past few days. They feel that more could be done in the policing area. And I just hope that the authorities do all they can."
The Assistant Chief Constable for Belfast, Mr Alan McQuillan, said he shared the Taoiseach's sense of frustration.
"Yes, we do know who these people are and we want to do everything we can to bring them to justice. There is no lack of determination among my officers to get the evidence needed to secure convictions," he told The Irish Times.
The Minister for Foreign Affairs, Mr Cowen, said the killing represented a "return to the brutality and futility of the past".
Those "clinging to hate-filled violence" must not be allowed to cast Northern Ireland back into darkness, the Northern Secretary, Dr John Reid, said.
"Another young man has had his life cruelly and brutally ended and another family has been devastated by evil people. Everyone in Northern Ireland has to confront the struggle between peace and hatred. It is a struggle people of goodwill in both communities must and will win," he added.
There was only so much the British government could do as long as it was acceptable for people to "live in a parallel universe where it is normal to go out and attack police officers", he told Sky's Sunday with Adam Boulton programme.
Mr McColgan's death was nothing short of "an appalling, cowardly murder", the PSNI Chief Constable, Sir Ronnie Flanagan, said on the same programme. While the arrest of two people in connection with the killing was "some progress in an absolutely tragic situation", the recent outpouring of sectarianism in north Belfast was downright "barbaric", he added.
The MP for the area, the DUP's Mr Nigel Dodds, said the young postman's killing could "not be justified under any circumstances . . . Such brutal murders not only visit terrible tragedy on the family of the victim but also increase fears and tensions throughout the entire community".
The SDLP's North Belfast MLA, Mr Alban Maginness, described Mr McColgan's death as a "dreadful and pointless murder" and the "almost inevitable consequence" of sectarian hatred and violence. "At the end of a sickening week, this is the most sickening event of all."
Appealing for calm, Sinn Féin president Mr Gerry Adams said the British government had so far failed to meet the challenge presented by the situation in north Belfast.
Mr McColgan's killing was further evidence of security force collusion with loyalist paramilitaries, the Sinn Féin MLA for North Belfast, Mr Gerry Kelly, claimed.
The Catholic Bishop of Down and Connor, Dr Patrick Walsh, who will conduct Mr McColgan's funeral tomorrow, described his killing as "clearly premeditated sectarian murder".
The Northern Ireland Workers' Party said those responsible served "no cause other than blind sectarian hatred".