Barack Obama has had a private meeting with families of the 14 people killed in the shooting attack in San Bernardino, hailing them as representative of the "unity and love" of America.

The US President said he was moved by their emphasis on tolerance following the massacre and that many wanted to know how such tragedies could be prevented from occurring in the future.

The President, accompanied by the First Lady Michelle Obama, stopped off in California en route to Hawaii for his Christmas holiday.

On arriving at San Bernardino airport on Air Force One they were greeted by the town's Mayor Carey Davis.

The couple were cheered by residents who lined the streets to cheer the presidential motorcade.

"For the President of the United States to come here and support them? It's amazing. It's just amazing. They can't believe it," said one resident.

Speaking after meeting the families, Mr Obama said: "It was so moving for Michelle and myself, in part because it was so representative of the country. You had people from every background, every faith.

"And, as difficult as this time is for them and for the entire community, they also represent the strength and the unity and the love that exists in this community and in this country."

He added: "And, having met some of these folks, despite the pain and heartache that they're feeling, they could not have been more inspiring and more proud of their loved ones and more insistent that something good comes out of this tragedy.

"And, many of them are already taking initiatives to reach out, to speak out on behalf of the community and tolerance, treating people with respect.

"Many were interested in how we can prevent shootings like this from happening in the future.

"It was a reminder of what's good in this country and I hope that's something that gives all Americans a sense of pride and a sense of hope as we go into our celebrations of our faith and our families and our countries."

His visit came as the man who bought the assault rifles used by the couple responsible for the attack, Syed Farook and his Pakistani wife Tashfeen Malik, was charged with terrorism-related offences for plotting an earlier attack that was aborted.

Enrique Marquez Jr, 24, who lived next door to Farook, who introduced him to Islam 10 years ago, was charged with conspiring to provide material support to terrorists.

Marquez told authorities he converted to Islam at the age of 16, and four years later was spending most of his time at Farook's home, listening and watching to "radical Islamic content," which included al Qaeda instructions on how to make bombs.