More than 20 police officers and their families have had to abandon their homes in Northern Ireland in the past six months because of fears that they were being targeted for assassination by dissident republicans.

Officers in the Police Service of Northern Ireland have been relocated to secret addresses because of intelligence showing that some republicans are preparing to take their campaign to a new level.

One of the three dissident groups, Óglaigh na hÉireann, is understood to have rearmed with up to 70 new rifles.

Security sources have told the Guardian that the intelligence branch of the PSNI, known as C3, has been working non-stop with MI5 to halt a number of suspected plots, and that surveillance of dissidents by police and the army is at its highest level for years.

The targeting of police officers in Londonderry and Belfast is causing most concern.

The Police Federation of Northern Ireland confirmed that more than 20 of its members had been forced to move home since the summer because intelligence reports indicated they were being targeted by one or more of the three dissident groups.

A spokesman for the Police Federation said: "There is no doubt about it, there are at least 20 if not more members who have been forced to leave their homes. There are even more officers who have temporarily moved out of their houses because of terrorist targeting.

"We know that the PSNI's high command is extremely anxious about the vulnerability of officers and their families. It is the rank and file that is presently feeling the pressure."

The spokesman added that the dissident organisations – ONH, the Real IRA and Continuity IRA – had been boosted by an influx of what he called "disillusioned ex-Provos" into their ranks.

All three groups are thought to have targeted a number of prison officers, several of whom have had to be moved to other locations.

The intensification of the dissident campaign coincides with a political crisis within the Northern Ireland power sharing executive.

Sinn Féin has threatened to leave the coalition with unionists if the Democratic Unionist party continues to block the transfer of policing and justice powers to the assembly.

If the assembly and the power-sharing institutions collapsed, republicans opposed to the peace process would portray the political failure as a victory for them.

In Belfast the police and security services have discovered an ONH plan to shoot dead PSNI officers travelling in non-armoured patrol cars. The plot involved a sniper attack from Milltown Cemetery on a police patrol car passing along the M1.

"Óglaigh na hÉireann have drawn up plans to kill policemen in Milltown in west Belfast. They are sitting ducks there at the moment," one security source said this week.

Two senior members of ONH have also been spotted scouting the area around the graveyard – a sacred place for republicans, where many IRA members are buried – in taxis over the past few weeks, the source said.

ONH has been significantly rearmed after its members bought up to 70 deactivated guns from the gun dealer Billy Bell. Bell killed himself in October 2008 after he discovered he had terminal cancer. The security forces now believe that before Bell died, he reactivated the rifles and sold them on to an ONH member from the Andersonstown area of west Belfast.

ONH first emerged in 2005 after divisions within the Real IRA. It has units in north and west Belfast as well as East Tyrone.

It has within its ranks experienced bomb-makers, one of whom constructed an under-car device that almost killed the girlfriend of a PSNI officer when it exploded near his home in east Belfast last month. ONH was also responsible in November for leaving a car bomb close to the headquarters of Northern Ireland's policing board at Clarendon Dock in central Belfast.

Dissident republicans have also used the tactic of shooting young men accused of anti-social behaviour to drum up support in the nationalist community. Just before Christmas the Continuity IRA admitted responsibility for shooting a young man in west Belfast who had served a prison sentence for the murder of a local shopkeeper, Harry Holland.

Violent year of dissident republican activity

7 March - Real IRA gunmen shoot dead army sappers Patrick Azimkar and Mark Quinsey outside the Massereene army base in Antrim town. Two pizza delivery men are seriously wounded in the shooting. The two soldiers were due to fly out to Afghanistan the following day.

10 March - Continuity IRA sniper shoots dead Constable Stephen Carroll in Craigavon, County Armagh. Carroll is the first Police Service of Northern Ireland officer to die at the hands of republican dissidents.

April - Northern Ireland's deputy first minister and Sinn Féin member, Martin McGuinness, says there is a republican dissident plot to assassinate him following his denunciation of the Real and Continuity IRA as "traitors" to the Irish people.

August - Heavily armed Real IRA unit takes over the Armagh village of Meigh and hands out leaflets urging local people not to co-operate with the PSNI.

November -Óglaigh na hÉireann attempts to blow up Policing Board headquarters in the centre of Belfast.

December - A high court judge is forced to leave his home in north Belfast following threats to his life from dissident republicans. In the same month, the then governor of Maghaberry prison, Steve Rodford, flees Northern Ireland after personal details about him are found in the cell of a republican prisoner at the jail.