But two unionist parties said they would boycott the meetings in protest at the presence of Sinn Fein.

The Irish premier has been working behind the scenes

Mr Ahern said he was optimistic over the talks at Stormont, near Belfast, despite the non-attendance of the Democratic Unionist Party and the UK Unionists.

He hoped progress could be made on the main sticking point - the issue of paramilitary disarmament.

After the murder of a Catholic man in Belfast at the weekend by a breakaway loyalist group, Northern Ireland's politicians lodged a renewed commitment to finding a solution.

Ireland Correspondent Mark Devenport: "Ahern has his work cut out"

Warnings have been issued that a new spate of sectarian murders could follow the killing.

Billy Hutchinson, of the Progressive Unionist Party, said he feared for the safety of people in north Belfast.

The parties missed Saturday's deadline for moving towards the establishment of a cross-border shadow executive.

Officials said Mr Ahern's determination to make moves on the accord struck last Easter had been intensified by the murder of Brian Service.

He underscored "the critical importance" of ensuring that those who were only interested in violence did not once again seize the initiative in Northern Ireland.

Blair may visit

With Mr Ahern in Belfast are Foreign Minister David Andrews and his deputy Liz O'Donnell. All three were involved in the negotiations that led to the deal.

First Minister David Trimble and his deputy Seamus Mallon met the Democratic Unionist Party and the UK Unionists before the visit by Mr Ahern.

The two parties indicated they would not attend the round-table discussions with Mr Ahern.

Peter Robinson, the DUP deputy leader, said after his meeting with the assembly leaders that he believed preparations were being made for a visit by Tony Blair in the near future in an attempt to crack the deadlock.

Mr Trimble has refused to establish the assembly's executive until the IRA starts disarming, and with no sign of any movement on a weapons hand-over, progress has ground to a halt.

Mr Mallon said the killing reinforced the need for speedy political progress.

He said he hoped the involvement of Mr Ahern would help break the impasse and focus political minds.