The Police Service of Northern Ireland said the so-called "horizontal mortar" _ an Irish Republican Army innovation developed by the outlawed group's engineers in the early 1990s _ was discovered thanks to a telephoned tip-off in the city of Armagh.

The IRA designed the device, when concealed at a roadside, to fire a mortar shell sideways at point-blank range into a passing police or army vehicle. The devices were used to devastating effect in the years running up to the IRA's 1997 cease-fire but have caused no serious casualties in the past decade of sporadic violence by IRA splinter groups.

Police Chief Superintendent Alasdair Robinson said whoever planted the Armagh weapon intended to strike a passing police patrol.

The device was left in a predominantly Catholic part of Armagh that suffered summertime rioting stoked by IRA dissidents. The die-hards _ rooted in Northern Ireland's Catholic minority _ previously have planted several other roadside bombs in or near Armagh in recent years, but all have been detected before they could go off.

Thursday's discovery of a sophisticated homemade mortar system offered the latest evidence that veterans of the IRA's 1970-1997 campaign were offering technical advice, at least, to the dissidents still trying to keep the IRA campaign alive.

Earlier this month, an expert panel that advises the British and Irish governments on dissident IRA activity published a report that said the breakaway groups' activities and skills were growing, in part, because of help from a handful of experienced IRA members.

Two dissident groups, the Real IRA and Continuity IRA, seek to wreck the cease-fire and the Catholic-Protestant government in Northern Ireland inspired by the peace process.

They have failed to kill anybody since March, when the Real IRA shot to death two off-duty British soldiers as they collected pizzas and the Continuity IRA shot a policeman fatally through the back of the head as he sat in his patrol car.