As relatives mark the fifth anniversary of the Omagh bombing, politicians and police officers have vowed to bring those responsible to justice.

A ceremony was held near where the car bomb exploded in 1998, killing 29 and injuring more than 200. It was the worst single attack in the history of the troubles in Northern Ireland.

Only one man has been convicted in connection with the atrocity so far. Colm Murphy was sentenced to 14 years imprisonment in January 2002 for plotting the attack.

Speaking to the BBC Radio 4 Today programme on Friday, Ulster minister Jane Kennedy said she was "confident" of further convictions, but declined to offer a timescale.

"The police remain confident that their investigation will be successful," she said.

"I was briefed only early this week in detail by the assistant chief constable responsible for the oversight of that investigation. It is one of the largest investigations in the UK. It continues as a live investigation.

"Obviously, there have been great frustrations for the police, for the families of those involved and for the town of Omagh but, notwithstanding that, the investigation continues and continues very vigorously."

"Five years on from that atrocity, primarily today is about commemorating those who lost their lives in that event and those many more who were injured," she added.

The Police Service of Northern Ireland also insisted that inquiries were continuing.

"We are disappointed that we have arrived at the fifth anniversary without a successful prosecution but this is still very much a live investigation," said a spokesman.

Last week, the government announced that £800,000 would be contributed towards a civil action against those believed to be responsible for the bombing.

The move was welcomed by the Conservatives, but marking the anniversary the party argued that public prosecutions were also needed.

"Five years on from the Omagh bombing we salute the unstinting courage and resolution of the Omagh victims' families in their efforts to bring to justice the perpetrators of this terrorist atrocity," said shadow Northern Ireland minister Lord Glentoran.

"Everyone in society has a duty to ensure that those who planned and carried out Omagh are never in a position to commit such crimes again. Justice needs to be done and to be seen to be done. Until that happens we should not rest easily."