Security forces on both sides of the border are hunting for a large bomb smuggled into Northern Ireland, according to reports.
"Intelligence traffic" reportedly indicates a plot to explode a bomb in Ulster, and that the device had already been transported across the border from the south in a car.
One report said the bomb was made by a former Provisional IRA bomb-maker from Downpatrick.
"The red light went up on Monday and there is a panic on that the next thing to happen is a bomb somewhere in the north," a veteran security officer told one newspaper.
"The problem is that no one either in the PSNI or the Garda appears to have specific intelligence where it is destined for."
The 300lb device would be similar to a car bomb abandoned outside a school in Castlewellan, County Down, last month.
A spokesman for the PSNI refused to confirm or deny the reports, adding: "We do not comment on intelligence matters."
The bomb scare comes as heads of the two police forces meet in Belfast to discuss recent violence by dissidents.
Commissioner Fachtna Murphy and other senior officers from the Republic of Ireland have discussed better coordination of anti-terrorist activities with Chief Constable Hugh Orde, commander of the Police Service of Northern Ireland.
Their talks at Orde's headquarters in east Belfast focused on intensified surveillance and eavesdropping of members of two splinter groups, the Real IRA and Continuity IRA, which have claimed responsibility for the past week's fatal shootings of two soldiers and a policeman.
After the meeting, Orde said both police forces were "determined to bring those responsible for these despicable murders over the last week to justice."
"An attack on any police officer is an attack on the whole of society," said Murphy, who pledged his force's "continued support in tackling and frustrating this small group of criminals."
Both breakaway groups are strongest along Northern Ireland's 225-mile (360-km) border with the Irish Republic — a frontier that is normally wide open now because of Northern Ireland's largely successful peace process.
The Republic of Ireland's police force, the Garda Siochana, said three of its elite squads — the National Surveillance Unit, Emergency Response Unit and Special Detective Unit — had deployed officers to the border, where they were providing shadowy backup to an overt show of strength designed principally to reassure the public.
Since Tuesday, police on the Republic of Ireland side have been stopping and searching vehicles bound for Northern Ireland. The searches cover only the biggest of more than 100 cross-border roads.
Thursday's security strategy conference occurred against a backdrop of grief in Northern Ireland's security forces.
The British Army planned a memorial service Thursday afternoon in honor of two Corps of Royal Engineers soldiers — Cengiz "Patrick" Azimkar, 21, and Mark Quinsey, 23 — who were gunned down Saturday outside their army base as they collected pizzas from delivery men. They were the first soldiers killed in Northern Ireland since 1997. The Real IRA claimed responsibility for that ambush, which also wounded two other soldiers and both pizza couriers.
And hundreds of mourners arrived at the home of Constable Stephen Carroll, 48, to attend his wake. Carroll, an English-born Catholic, was shot through the head Monday as he sat in his patrol car. The Continuity IRA has said it killed him.
Before the past week's killings, Orde repeatedly warned that his anti-terrorist detectives believed a rise in dissident violence was imminent. Since November 2007, both dissident groups have mounted more than 20 gun, bomb and rocket attacks on police, wounding seven — but killing none until Monday.
Carroll was the first police officer killed in Northern Ireland since 1998, the year rival British Protestant and Irish Catholic politicians struck the Good Friday peace accord. Today's dissidents hope to unravel that deal, which inspired the IRA's 2005 decision to disarm and 2007's formation of a joint Catholic-Protestant government.
The dissidents have not detonated a car bomb in Northern Ireland since August 1998, when the Real IRA killed 29 people, mostly women and children, on a crowded street in the town of Omagh.
But they have tried repeatedly in recent years. More than a half-dozen car bombs have failed to do damage because of faulty construction, police interception or dissident second thoughts.
In the most recent threat, dissidents abandoned a 300-pound (150kg) car bomb Jan. 26 several miles (kilometers) short of their target, a British Army base, citing unexpected police checkpoints on the road. Army explosives experts said that fertilizer-based bomb was more expertly constructed than the dissidents' previous failures, most of which involved ill-judged attempts to detonate fuel cylinders.