The New York Times has published a front page editorial for the first time since 1920, calling for an end to the gun violence that continues to plague the United States.

Key points: New York Times publishes front page editorial calling for end to gun violence

New York Times publishes front page editorial calling for end to gun violence First time the paper has published an editorial on the front page since 1920

First time the paper has published an editorial on the front page since 1920 More than 300 mass shootings in America in 2015, tracking website says

More than 300 mass shootings in America in 2015, tracking website says More than 12,000 deaths due to gun violence

The editorial said laws that allowed the legal purchase of high-powered assault weapons in the country were a "moral outrage and a national disgrace".

"America's elected leaders offer prayers for gun victims and then, callously and without fear of consequence, reject the most basic restrictions on weapons of mass killing," the editorial said.

"They distract us with arguments about the word terrorism. Let's be clear: These spree killings are all, in their own ways, acts of terrorism."

The editorial's publication came after the mass shooting at a social services agency in San Bernardino, California, on Thursday, which the FBI has since declared an "act of terrorism".

The shooters, Syed Rizwan Farook, 28, and his wife Tashfeen Malik, 27, were armed with assault weapons and semi-automatic handguns when they burst into an auditorium rented out for a Christmas party at the Inland Regional Centre.

The pair shot dead 14 people and left 17 others injured.

Islamic State has claimed responsibility for the attack, but US government said earlier there was no evidence the shooting was directed by the militant group or that it even knew who the attackers were.

"Two followers of Islamic State attacked several days ago a centre in San Bernardino in California," the group announced in an online radio broadcast.

The FBI said it lacked evidence that the married couple who carried out the attack belonged to a larger organisation of extremists.

President Barack Obama insisted that the US "will not be terrorised" and renewed his call for tighter gun control measures in his weekly address.

"It is entirely possible that these two attackers were radicalised to commit this act of terror," he said.

US investigators said they were evaluating evidence that Malik, a Pakistani native who had been living in Saudi Arabia when she married Farook, had pledged allegiance to IS leader Abu Bakr al-Baghdadi.

"There's a number of pieces of evidence that has essentially pushed us off the cliff to say we are now investigating this as an act of terrorism," FBI assistant director David Bowdich said.

A Los Angeles Times report citing a federal law enforcement officer said Farook had contact with people from Al Nusra Front, an Al Qaeda affiliate in Syria.

The official also said Farook had made "some kind" of contact with the radical Al Shabaab group in Somalia. It is unclear what type of contact or with whom, the newspaper said.

Two assault-style rifles, two semi-automatic handguns, 6,100 rounds of ammunition and 12 pipe bombs were found in the couple's home or with them when they were killed, officials said.

Publisher wanted 'strong and visible' statement

Arthur Sulzberger, Jr, the Times' publisher, said the editorial was intended "to deliver a strong and visible statement of frustration and anguish about our country's inability to come to terms with the scourge of guns".

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"It has been many decades since The Times ran an editorial on Page One," he said in a statement.

"Even in this digital age, the front page remains an incredibly strong and powerful way to surface issues that demand attention. And, what issue is more important than our nation's failure to protect its citizens?"

The Times editorial said the US Constitution's Second Amendment — which protects the right of US citizens to bear arms — should not be immune from "reasonable regulation".

"Certain kinds of weapons, like the slightly modified combat rifles used in California, and certain kinds of ammunition, must be outlawed for civilian ownership," the editorial said.

"It is possible to define those guns in a clear and effective way and, yes, it would require Americans who own those kinds of weapons to give them up for the good of their fellow citizens."

Over 300 mass shootings in the US this year: monitor

Angala Cariou (R) and her daughter Brittany Cariou visit a makeshift memorial near the Inland Regional Centre ( AFP: Justin Sullivan )

There have been 353 mass shootings in America in 2015, according to Mass Shooting Tracker, a website which records data of shootings in the United States.

The site defines a mass shooting as when "four or more people are shot in an event, or related series of events, likely without a cooling off period".

There have been 12,281 deaths and 24,803 injuries due to gun violence in the United States this year according to the Gun Violence Archive, a not-for-profit organisation that publishes information about gun violence.

Last month, a gunman stormed a Planned Parenthood clinic in Colorado Springs, killing three people and wounding nine others.

In October, gunman Chris Harper-Mercer shot dead 10 people and wounded seven others at Umpqua Community College campus in Roseburg, Oregon.

At the time, Mr Obama said: "We've become numb to this ... I can imagine the press releases being cranked out — we need more guns, they'll argue. Fewer gun safety laws. Does anybody really believe that?"

"Motives do not matter to the dead in California, nor did they in Colorado, Oregon, South Carolina, Virginia, Connecticut and far too many other places," the Times editorial said.

The last time the Times ran a front page editorial was on June 13, 1920, when Warren Harding was nominated as the Republican presidential candidate.

The New York Times editorial in full:

All decent people feel sorrow and righteous fury about the latest slaughter of innocents, in California. Law enforcement and intelligence agencies are searching for motivations, including the vital question of how the murderers might have been connected to international terrorism. That is right and proper. But motives do not matter to the dead in California, nor did they in Colorado, Oregon, South Carolina, Virginia, Connecticut and far too many other places. The attention and anger of Americans should also be directed at the elected leaders whose job is to keep us safe but who place a higher premium on the money and political power of an industry dedicated to profiting from the unfettered spread of ever more powerful firearms. It is a moral outrage and a national disgrace that civilians can legally purchase weapons designed specifically to kill people with brutal speed and efficiency. These are weapons of war, barely modified and deliberately marketed as tools of macho vigilantism and even insurrection. America's elected leaders offer prayers for gun victims and then, callously and without fear of consequence, reject the most basic restrictions on weapons of mass killing, as they did on Thursday. They distract us with arguments about the word terrorism. Let's be clear: These spree killings are all, in their own ways, acts of terrorism. Opponents of gun control are saying, as they do after every killing, that no law can unfailingly forestall a specific criminal. That is true. They are talking, many with sincerity, about the constitutional challenges to effective gun regulation. Those challenges exist. They point out that determined killers obtained weapons illegally in places like France, England and Norway that have strict gun laws. Yes, they did. But at least those countries are trying. The United States is not. Worse, politicians abet would-be killers by creating gun markets for them, and voters allow those politicians to keep their jobs. It is past time to stop talking about halting the spread of firearms, and instead to reduce their number drastically - eliminating some large categories of weapons and ammunition. It is not necessary to debate the peculiar wording of the Second Amendment. No right is unlimited and immune from reasonable regulation. Certain kinds of weapons, like the slightly modified combat rifles used in California, and certain kinds of ammunition, must be outlawed for civilian ownership. It is possible to define those guns in a clear and effective way and, yes, it would require Americans who own those kinds of weapons to give them up for the good of their fellow citizens. What better time than during a presidential election to show, at long last, that our nation has retained its sense of decency?

ABC/wires