Two soldiers have been shot dead during a gun attack at an army base in County Antrim, the Ministry of Defence says.
A spokesman said "four other personnel" were injured, one of them critically, in the attack at Massereene army base in Antrim, 16 miles north of Belfast.
No one has said their gunmen carried out the attack, but it is thought to be the work of dissident republicans.
Northern Ireland Secretary Shaun Woodward condemned the shootings as "an act of criminal barbarism".
Earlier reports suggested all those killed and injured were male. The MoD said the next of kin of the dead soldiers had been informed.
Mr Woodward added: "My thoughts are with the families of those killed and injured in this murderous attack.
"The contrast between those who serve the community and those who would destroy it could not be clearer. The people who did this will be pursued and they will never stop the political process in Northern Ireland."
The attack was being investigated by the Police Service of Northern Ireland, the statement added.
The soldiers are the first to be murdered in Northern Ireland since Lance Bombardier Stephen Restorick was killed by an IRA sniper in 1997.
All four injured men have been taken to Antrim Area Hospital, about a mile away from the scene.
The condition of some were said to be serious.
It is believed that there were two long busts of gunfire during the incident.
A major security operation is under way and the area surrounding the barracks, which is home to 38 Engineering Regiment, has been sealed off.
Eyewitness reports have suggested the victims were taking a pizza delivery when the shooting began.
One witness who lives near the base told the BBC how he looked to the sky after hearing what he thought were fireworks.
For the last 10 years, people believed things like this happened in foreign countries... Unfortunately it has returned to our doorstep
He added: "Then I heard a lot of loud bangs again, only it was a lot more than there was initially - maybe between 10 and 20.
"Then the siren at the army barracks went off. Then all you heard was the police sirens and ambulances and there was at least six ambulances.
"There was definitely six of the ambulances and God knows how many police cars - they just came out of the police station one after the other."
In a statement, Downing Street said everything possible would be done to ensure those responsible would be brought to justice.
"This is a terrible incident that we utterly condemn and the prime minister's thoughts, first and foremost, are with the families of those killed and with those seriously injured in this attack," it continued.
Northern Ireland's First Minister and Democratic Unionist Party leader Peter Robinson offered his sympathies to the families of the victims, and said he would postpone a scheduled trip to the United States.
Mr Robinson said the attack was "terrible reminder of the events of the past".
Rev Dr William McCrea at Massereene Barracks
He added: "These murders were a futile act by those who command no public support and have no prospect of success in their campaign. It will not succeed."
David Ford, the leader of the Alliance Party, said it was "inconceivable" that the attack could have been carried out by anyone other than dissident republicans.
"The important thing is that we must not see politics and the peace settlement that we have, fragile though it is, destabilised by this kind of action," he added.
The attack comes shortly after Sir Hugh Orde, the chief constable of the Police Service of Northern Ireland, requested the Special Reconnaissance Regiment's help to gather intelligence on dissident republicans.
He had said the threat against his officers and military personnel was at its highest for almost a decade.
Mr Robinson said the Massereene attack vindicated his decision, which had been criticised by Sinn Fein.
In 2008, dissident republicans attempted to kill PSNI officers during separate incidents in Derry City and Dungannon, Co Tyrone.
Security forces defused a 300lb (136kg) bomb in Castlewellan, Co Down, close to a barracks in February 2009.
Ian Paisley Jnr, a DUP member of the Northern Ireland Assembly and the Policing Board, said the shooting could prove to be a defining moment in Northern Ireland's history.
He said: "For the last 10 years, people believed things like this happened in foreign countries, places like Basra. Unfortunately it has returned to our doorstep."
The leader of the nationalist SDLP, Mark Durkan, condemned the "murderous" attacks.
"Those who committed it are steeped in the mindset and means of past violence," he added.
"They need to understand this is not an attack on British army but the Irish people who have voted for and value above all else peaceful politics and democratic accommodation."
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