Two military personnel have been shot dead during a gun attack at an Army base in County Antrim, police said.

The Police Service of Northern Ireland said two further military personnel and two civilians, all believed to be male, were were in a serious condition.

The incident took place at the Massereene Army base in Antrim, 16 miles north of Belfast, at 2140 GMT.

Northern Ireland Secretary Shaun Woodward condemned the shootings as "an act of criminal barbarism".

He added: "My thoughts are with the families of those killed and injured in this murderous attack."

A spokesperson for the Ministry of Defence described the incident as a "drive-by" shooting.

'Loud bangs'

The four injured men have all been taken to Antrim Area Hospital, about a mile away from the scene.

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.

One witness who lives near the base told the BBC how he looked to the sky after hearing what he thought were fireworks.

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. For the last 10 years, people believed things like this happened in foreign countries... Unfortunately it has returned to our doorstep

"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."

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".

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."

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."

SDLP Assembly member Thomas Burns condemned what he called a "particularly brutal and horrific attack" and urged anyone with information to contact the police.

He said: "Like many, we had hoped and prayed that nights like this were firmly in the past. It is important we unite as a community and send out a message to those responsible that they have absolutely no support."

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.

No-one has so far claimed responsibility.

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.

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.