President Barack Obama has signed into law a $633 billion US 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 Thursday after signing the measure.

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

"In a time when all public servants recognize the need to eliminate wasteful or duplicative spending, various sections in the Act limit the Defense Department's ability to direct scarce resources towards the highest priorities for our national security," the president said.

"Even though I support the vast majority of the provisions contained in this Act... I do not agree with them all," he said in his statement, adding that he did not have the constitutional authority to approve piecemeal items within the sprawling bill.

"I am empowered either to sign the bill, or reject it, as a whole," he said.

The meaure was hammered out by House and Senate conferees last month after each chamber voted to approve separate versions of the bill.

It includes $527.4 billion for the base Pentagon budget; $88.5 billion for overseas contingency operations including the war in Afghanistan; and $17.8 billion for national security programs in the Energy Department and Defense Nuclear Facilities Safety Board.

The bill authorizes $9.8 billion for missile defense, including funds for a Pentagon feasibility study on three possible missile defense sites on the US East Coast.

It also extends for one year the restriction on use of US funds to transfer Guantanamo inmates to other countries, a limitation critics say marks a setback for Obama's efforts to close the detention center.