The PSNI Chief Constable Simon Byrne has said police believe information about more than 10,000 officers and staff, mistakenly released in a major data breach last week, is now in the hands of dissident republicans.
Chief Constable Byrne said: "It's therefore our planning assumption that they will use this list to generate fear and uncertainty as well as intimidating or targeting officers and staff.
"I won't go into detail for operational reasons, but we are working around the clock to assess this risk and take measures to mitigate it."
He said that contrary to some media reports there has not been a movement of police officers or staff out of the organisation.
He added: "We have been strongly supported by a range of cyber specialists from across the policing system in dealing with this unprecedented incident.
"We have measures in place to reassure and advise our workforce of what this risk means for them. We will continue to liaise with the policing board and the UK government as well as other partners as we develop our response to this matter."
He said "the safety and welfare of all our officers and staff remains my top priority".
Police Federation chairman Liam Kelly said it was a "deeply worrying development" that dissident republicans had information from the data breach.
The Police Federation of Northern Ireland represents rank and file PSNI officers.
Mr Kelly said: "Our men and women are resilient and resourceful. They must call on all their training and professionalism to counter this ugly consequence of a monumental data breach.
"Even though we believed from the outset the data would find its way into the hands of people intent on murdering or maiming our colleagues, it is obviously a deeply worrying development.
"We have to be strong and determined to do everything we can to minimise risk and that means varying the routes we take to and from work, changing routines and re-assessing our personal security both on and off duty.
"We can thwart the consequences of the violent dissident threat and rhetoric, but it will require an all-out effort by each and every one of us."
Northern Ireland Secretary Chris Heaton-Harris said the UK government remains committed to supporting the PSNI following the mistaken release of staff information.
"The UK Government remains committed to providing specialist support and expertise, and I know that PSNI and security partners will continue to take proportionate action to protect officers and staff and their families," Mr Heaton-Harris said.
"We will remain in close contact with the Chief Constable and his senior team," he added.
Earlier today, a Sinn Féin MLA said that dissident republicans posted redacted information from the data breach on the wall of a library in Belfast to prove they are in possession of the sensitive material.
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Gerry Kelly said the document which was posted overnight close to the Sinn Féin office on the Falls Road, included information about a "substantial number" of police officers and staff, although their names had been removed.
Policing Board member Mr Kelly, who described the posting of the information as sinister, explained that he had been contacted by a party member this morning who had arrived at the office.
He said: "He noticed that on the side of the library there was a number of documents pasted up.
"There was a photograph of myself and then there was a statement saying in large writing: 'Gerry, we know who your mates are.'
"Under that there was what appeared to be, and what turned out to be, a section of the leaked documents that were put out.
"It did not have the names of the police officers involved, but it had everything else.
"It wasn't the whole 10,000 (names), but it was a substantial number."
Mr Kelly added: "I look upon this as a threat by dissidents to me and I will not be intimidated.
"More serious is that this is the dissidents, or whoever is involved, putting out that their claim that they have access to the leaked documents, they are putting out a verification on that.
"I think that is their main intention."
Asked why he believes dissident republicans are responsible, Mr Kelly said: "To have the names of the officers, if you were caught putting such information up then you would be under criminal charge immediately.
"One of the reasons that makes me believe it is dissidents is precisely that.
"They were clever enough to remove and to put up what they believe would not be illegal."
Assistant Chief Constable Chris Todd confirmed that an investigation has begun into the posting of the redacted information in Belfast.
He said: "We are aware that some redacted information from the Tuesday August 8 data breach has allegedly been posted on a wall in west Belfast today, Monday August 14.
"We have commenced an investigation into this matter.
"From the outset we have been planning for this potential development and that plan is now being put into place."
Mr Todd continued: "We recognise the impact this may have on our officers, staff and their families and additional security and reassurance patrols have already been implemented across Northern Ireland as part of our organisational response.
"The safety and welfare of our officers and staff remains our priority and we have reminded them of their personal safety and security both on and off duty."
Hundreds of officers have expressed concern for their safety in Northern Ireland, where police are under threat from terrorists - with the current level of threat assessed as severe, meaning an attack is highly likely.
The Police Federation of Northern Ireland said that by this afternoon 2,905 officers had expressed an interest in the event of legal action following the major data breach.
While no legal action has yet commenced, it is thought officers and staff whose names and places of work were revealed could be in line for compensation.
The PSNI is already facing significant budget pressures and any settlements would be expected to run into tens of millions of pounds.
Additional reporting: PA
