Political tension has escalated dramatically in Northern Ireland, with the second fatal attack on the security forces within days.
A police officer was shot dead while on patrol on Monday, two days after the fatal shooting of two British soldiers.
The officer was lured into an ambush in Craigavon, in County Armagh, about 40km from the capital Belfast.
It was the first killing of a serving police officer in the British province for 10 years.
Unconfirmed sources say two police officers were fired on as they responded to what they believed to be a call from someone who lives on the Lismore Manor Estate in Craigavon.
It is understood police were shot at from an area of waste ground on the estate, the BBC reports. One officer has since died. No one has yet claimed responsibility for his death.
There is now a heavily armed police presence. Roads have been closed as a large scale search of the area is carried out.
Meanwhile, British Prime Minister Gordon Brown says the killing of two soldiers by gunmen opposed to Northern Ireland's peace process would not drag the province back into violence.
The shooting of the soldiers at the Massereene Barracks in Antrim was the first of its kind in 12 years.
Mr Brown has visited an army base where gunmen from the Real IRA group killed the soldiers.
After decades of fighting, the IRA and Protestant guerrilla groups agreed to ceasefires under the Good Friday peace deal in 1998. More than 3600 people were killed in Northern Ireland from the late 1960s.
The Real IRA wants an end to British rule.
Northern Ireland garrison commander, Brigadier George Norton, said there are no plans to put troops back onto the streets of Northern Ireland.
The soldiers were the first troops to be killed in the province since 1997. They were named as Cengiz Azimkar, 21, from north London and Mark Quinsey, 23, from Birmingham.