(Updates with final vote total, background.)
By Patricia Zengerle
WASHINGTON, June 18 (Reuters) - The U.S. Senate passed an annual defense policy bill on Thursday that authorizes some $600 billion in defense spending for the 2016 fiscal year and starts reforms that could help curb costs over the long run.
The vote was 71-25, with the "no's" coming mostly from Democrats and the "yes" votes mostly from Republicans, who hold a majority of seats in the Senate.
To become law, it must be reconciled with a version of the bill passed by the House of Representatives last month, and then signed by Democratic President Barack Obama.
The White House has threatened to veto the measure over issues including a Republican-led plan to use $38 billion in special war funds to let the Department of Defense sidestep mandatory spending caps.
The White House is also unhappy with language in the Senate bill that would make it more difficult to close the military detention center at Guantanamo Bay in Cuba and lawmakers' refusal to adopt cost-cutting proposals including additional base closures and the retirement of the Air Force's A-10 "Warthog" close-air support aircraft.
Many of the same Senate Democrats who backed the authorization bill have joined the party's pledge to block every appropriations bill unless Republicans agree to start negotiations on a way to lift the spending caps for the entire federal government, not just the Pentagon.
Authorization bills give congressional approval for government programs. Appropriations bills provide the money to carry out those programs. (Additional reporting by David Alexander; Editing by Sandra Maler)