Northern Ireland's First Minister has said the raising of the threat level to Britain from dissident republicans is "worrying".
Last night, the British Home Secretary Theresa May said the threat level had been raised from "moderate" to "substantial" and that Irish-related attack is now a "strong possibility".
Northern Ireland's First Minister Peter Robinson said the move underlined the need for adequate police resources to tackle the threat.
He said dissident republicans had no widespread support.
The UK threat assessment was increased after the head of the MI5 warned that dissident Irish republicans could attempt to mount a new wave of terrorist attacks on the British mainland.
But it was still lower than the overall threat to the UK from international terrorism, published by the Joint Terrorism Analysis Centre (Jtac), which remains at severe, meaning an attack is highly likely.
Dissident republican groups remain small and isolated organisations, but experts warn they are growing in size and expertise.
The fringe groups are violently opposed to the peace process and have focused their energies on launching sporadic waves of attacks, primarily on the police.
And key figures in the North concede that it only takes a handful of dangerous individuals to inflict major casualties.
The murder of three security force members last year has been followed by a string of other attacks, as well as near misses where large-scale civilian, military or police deaths could easily have been caused.
The official paramilitary watchdog, the Independent Monitoring Commission (IMC), has said it is difficult to place a figure on the number of active dissidents.
But while the groups are small scale in comparison with the mainstream IRA and loyalist groups active at the height of the Troubles, there are early signs of some co-operation between the separate dissident republican organisations.
Prominent Sinn Féin member and former IRA prisoner Gerry Kelly, who is now a Junior Minister in the North’s power-sharing government, recently observed: “These small groups have the ability to do damage. Two or three people can do a lot of damage if they go undetected and they have the expertise.”
Experts have said the dissidents remain a loose collection of dangerous organisations with no central command.
But they have noted a worrying recruitment trend in recent years.
Professor Adrian Guelke, an expert on Irish terror groups at Queen’s University Belfast, said: “Throughout 2009 there was seen to be an increase in recruitment. Their capacity is quite advanced as they have recruited people who were former provos (members of the Provincial IRA). There is now concern that these groups have access to a team of experts.”
The main groups include:
:: The Continuity IRA: It has its origins in ideological divisions in the republican movement that forced splits in the 1980s. In 1986 Sinn Féin voted to change its party policy, opting to seek to take seats in the Dáil. This saw traditionalists, who refuse to accept the legitimacy of the Republic, leave its ranks to form the Republican Sinn Féin party. It has regularly denied being linked to the Continuity IRA (CIRA) paramilitary group that later appeared, but they are widely seen as being connected. The Continuity group came to wider attention with attacks in the mid-1990s that took place as tensions grew within republican ranks over the emerging peace process and the mainstream movement’s advance towards a solely political path. The Continuity group is strongest in pockets of the border counties. In March last year it claimed responsibility for the murder of PSNI constable Stephen Carroll in Co Armagh.
:: The Real IRA: The “RIRA” group has become the most prominent of the dissident organisations and was responsible for the Omagh bombing of 1998.
The blast claimed the lives of 29 people, including a woman pregnant with twins. The attack in the Co Tyrone town was the single biggest loss of life of the Troubles and while it forced the dissidents to wind-down their violence, the move proved only temporary.
The paramilitary group shot dead two soldiers outside Massereene Army base in Co Antrim last year, and has been linked to a string of bomb and gun attacks. It emerged from bitter splits inside republicanism over the peace process in the late 1990s and key members of the Provisional IRA broke away to form their own smaller armed group.
It was to become the biggest of the dissident organisations, and though it has overtaken the CIRA, the IMC believes the two have cooperated.
:: Oglaigh na hEireann: The IMC has described it as a small but dangerous group, bent on violence and involved in other criminal enterprises. It is also said to include former Provisional IRA members and has cooperated with other dissidents.
:: Republican Action Against Drugs: Is believed to be made up of dissidents who have carried out so-called punishment attacks in theDerry area. It may not be a stand-alone group, so much as a front for others.
:: The Irish National Liberation Army (INLA): It was formed in the mid 1970s and carried out some of the most notorious killings of the Troubles.
It was responsible for the 1997 murder of loyalist paramilitary leader Billy Wright, who was shot dead by two INLA prisoners inside the top security Maze prison.
Last week a report by a public inquiry, compiled at a cost of £30m (€35m), announced it had found no evidence of state collusion in the Wright killing despite theories to the contrary.
While the INLA opposes the Good Friday Agreement, it last year announced its campaign of violence was over and subsequently decommissioned weapons.