Barack Obama appealed to Americans to "come together as brothers and sisters" as he was joined by the First Family to switch on the lights on the National Christmas Tree - just a day after 14 people were shot to death in Southern California.

The president was joined by his wife Michelle, daughters Malia and Sasha, and his mother-in-law, Marian Robinson, as well as Hollywood actress Reese Witherspoon for the ceremony to jump start the festive season.

This year"s festive event, which also featured a variety of musical performances, is taking place under somber circumstances a day after a radicalized Muslim married couple killed 14 people in San Bernardino, California.

"This is of course the most wonderful time of the year, but we would be remiss not to take a moment to remember our fellow Americans whose hearts are heavy tonight, who grieve for loved ones, especially in San Bernardino, California," Obama said.

"Their loss is our loss, too. We"re all one American family," he added.

As a backdrop to the ceremony, the U.S. flag flew at half-staff atop the White House, on Obama"s orders, in remembrance of the victims.