A newspaper tonight received a call from a man claiming to be from the Real IRA taking responsibility for the murder last night of two soldiers shot dead in Northern Ireland, the first terrorist attack in the province for more than a decade.

A journalist at the Dublin-based Sunday Tribune's office in Belfast said that the newspaper took a call claiming responsibility for the attack by the Real IRA's Antrim brigade. The caller used a recognised codeword.

The attack at an army base in Co Antrim – in which the gunmen exploited a pizza delivery and were said to have "executed" soldiers lying on the ground after the initial attack – comes days after the Northern Ireland police service's chief constable, Sir Hugh Orde, said there was a growing threat of violence from dissident republicans. A huge police manhunt is now under way.

Speaking earlier today before the claim of responsibility, the Sinn Féin leader, Gerry Adams, called the attack "wrong and counter-productive" and, in an unprecedented statement, said his party had a "responsibility to be consistent ... the logic of this is that we support the police in the apprehension of those involved in last night's attack".

It was not until a vote at a special meeting of Sinn Féin in early 2007 that the party ended decades of opposition to the province's Protestant-dominated police force.

Gordon Brown condemned the killings as "cowardly", and vowed that they would not stop the peace process: "No murderer will be able to derail the peace process," he said.

Soldiers at the entrance to the Massereene army base in Antrim, 16 miles north of Belfast, were attacked last night by gunmen lying in wait in a vehicle as a pizza delivery was being made at 9.40pm. The gunmen were obviously aware that soldiers followed a regular routine on Saturday nights, when as many as 20 separate orders were made for pizza to be delivered. They waited until the troops emerged through the gates to pick up the food.

Reports said the gunmen raked the base with automatic fire then shot the soldiers as they lay injured on the ground. Two other members of the military were injured and two pizza delivery men, were also hurt, one of them, a 32-year-old Polish national, was in a critical condition.

It emerged today that the soldiers who were shot were dressed in desert fatigues and were due to leave on an early flight this morning for deployment to Afghanistan.

The names of the dead soldiers have been withheld until all their relatives have been informed. The soldiers were due to fly out of RAF Aldergrove, just a few miles away from their base in the early hours of today.

The plane, which had four empty seats, had to be held up until investigating police officers had finished interviewing soldiers about the attack.

Unionists said the attack brought Northern Ireland to a "defining moment" in its history. Unionist politicians will be studying the response from Sinn Féin, which may determine the prospects of the fragile power-sharing executive that has former enemies running Northern Ireland together.

Speaking about last night's attack, Adams said: "Those responsible have no support, no strategy to achieve a united Ireland. Their intention is to bring British soldiers back on to the streets. They want to destroy the progress of recent times and to plunge Ireland back into conflict ... the peace process was built against the odds and not least because of the willingness of republicans to take risks and to be strategic and long-sighted.

"There should be an end to actions like the one in Antrim last night. The popular will is for peaceful and democratic change."

Sinn Féin's Martin McGuinness, deputy first minister of Northern Ireland, said: "I was a member of the IRA, but that war is over now. The people responsible for last night's incident are clearly signalling that they want to resume or re-start that war. Well, I deny their right to do that."

Last night's attack was the first time British troops had been killed in Northern Ireland by terrorists since 1997. The last soldier to die before the IRA's second ceasefire was Lance Bombardier Stephen Restorick. More than 500 British soldiers died during nearly 40 years of conflict in Northern Ireland, known as the Troubles.

Chief Superintendent Derek Williamson said: "There is no doubt in my mind that this was an attempt at mass murder."

The scene of the attack is the current home of the 38 Engineer Regiment, which has only been based at Massereene barracks since August.

Nationalist and unionist politicians arrived at the scene of the shooting this morning on the outskirts of Antrim Town to condemn those responsible for the worst act of terrorism in the province since the Omagh bomb.

At midday, hundreds of churchgoers from churches across Antrim gathered at the police cordon near the murder scene to hold a prayer service. Traffic was halted as congregations from the Roman Catholic, Church of Ireland, Presbyterian and Methodist churches came together to pray for the victims.

Ian Paisley Jr, a member of the Northern Ireland Policing Board and Democratic Unionist assemblyman for North Antrim, said the province had reached a "defining moment".

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. "

In a swipe at Sinn Féin, who accused Sir Hugh of exaggerating the threat from dissident republican terrorists last week, Paisley added: "If this shooting is attributed to dissident republicans, then it was no exaggeration."

Sir Hugh told the policing board the threat level from anti-ceasefire republicans was "severe" and the highest since he had taken over running the police service.

Paisley's party colleague and Northern Ireland's first minister, Peter Robinson, today postponed a visit to the US alongside McGuinness, his deputy. Condemning the murders and the attempted murders, Robinson said they were a "terrible reminder of the events of the past".

Signalling the DUP's determination not to let the attack destabilise the province's power-sharing coalition, the first minister 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."

Speaking at the scene of the murder, Mark Durkan, the leader of the nationalist SDLP, said: "We know [the dissidents] are opposed to peace and we know their capacity to attack, threaten, disrupt and even kill.

"They need to understand that this is not an attack on the British army but the Irish people who have voted for and value above all else peaceful politics and democratic accommodation."

Since the summer security chiefs have been warning that dissident republicans were reorganising and upping their ability to launch attacks. Five PSNI officers have been wounded in separate gun and bomb attacks in Derry, Tyrone and Fermanagh. All police officers are once again wearing flak jackets.

There have also been a number of failed bomb attacks, one of which took place last month when the security forces found a 300lb (136kg) car bomb in Castlewellan, Co Down.

Security sources said the device "spooked" army technical officers and the wider security community because it was fitted with a type of sophisticated, secondary anti-handling device that they had not come across before.

Sir Hugh will hold a security summit later this week with his counterpart in the Irish republic, the Garda commissioner, Fachtna Murphy. Top of the agenda will be the dissidents' escalating campaign of violence.

• This article was amended on Monday 9 March 2009. Above we changed Stephen Restorick's rank from private to lance bombardier.