KANEOHE BAY, HAWAII: US President Barack Obama said the six US service members killed this week in a suicide attack in Afghanistan were "outstanding, brave men and women" as he delivered a Christmas Day gesture of gratitude to US troops for how much they sacrifice to keep Americans safe and free.

The six service members were killed Monday at Bagram Airfield in Afghanistan, the largest US facility in the country, when a suicide attacker rammed an explosives-laden motorcycle into a joint NATO-Afghan patrol. The Taliban claimed responsibility.

"As we know, when you're deployed overseas, it's tough," Obama said in brief remarks at Marine Corps Base Hawaii in Kaneohe Bay. He said that although his administration has been bringing home troops from Iraq and Afghanistan, "there are still folks over there every single day and it's still dangerous, as we saw this past week, where we had some outstanding, brave men and women who were killed."

"So we never take for granted what all of you do for the American people," Obama said. "You help keep us free. You help keep us strong. Whatever service you're in, whatever branch, we are extraordinarily grateful for everything that you do every single day."

The Christmas visit to the Marine Corps Base has become an annual tradition for Obama and his wife, Michelle, during the family's yearly vacation in his native Hawaii. The first lady and Jill Biden, the wife of Vice President Joe Biden, head a nationwide initiative to rally public support for active-duty and retired service members and their families.

Before the Friday afternoon visit, Obama opened Christmas gifts and sang carols with his family at their vacation rental in nearby Kailua, about a half hour from downtown Honolulu, the White House said.

Obama also spent part of Christmas Eve placing telephone calls to US service members from each branch of the military stationed around the world, the White House said. He thanked them and their families for serving and sacrificing for the country.

The president's quiet holiday morning followed a busy Christmas Eve in which he also exercised at the Marine Corps Base gym, took his daughters Malia and Sasha on a hike and to the beach. He capped the evening with a three-hour dinner with family and friends.

The White House didn't provide details on what gifts were exchanged, but Mrs. Obama spilled a few details before the family arrived on Oahu.

Mrs. Obama said during her annual holiday visit with hospitalised children that the president was getting "workout stuff." Her daughters were getting money.

"They don't want me to waste my energy picking out stuff they don't want," she said. "So they'd rather just have the money."

Meanwhile, Obama, who works out at the Marine Corps Base's gym just about every morning during vacation here, acknowledged feeling a tad inadequate exercising alongside Marines.

"I generally feel like your commander in chief is in pretty good shape, and then I get next to some guy, you know, curling 100 pounds and it makes me feel small," Obama said before wrapping up his remarks and posing for photographs with the service members. "But that's OK. It gives me some inspiration. I'm going to work harder so I can keep up with you next year."