US President Barack Obama has signed into law a $633 billion defense spending bill that funds the war in Afghanistan and boosts security at US missions worldwide.

"I have approved this annual defense authorization legislation, as I have in previous years, because it authorizes essential support for service members and their families, renews vital national security programs, and helps ensure that the United States will continue to have the strongest military in the world," Obama said in a statement early on Thursday after signing the measure.

Obama, who is vacationing in Hawaii, said that he signed the measure despite reservations.