(L-R) US President Barack Obama and US Secretary of Defense Ashton Carter speak before a briefing at the Pentagon on August 4, 2016 in Washington, DC. / AFP PHOTO / Brendan SmialowskiBRENDAN SMIALOWSKI/AFP/Getty Images

(L-R) US Vice President Joe Biden, US President Barack Obama and US Secretary of Defense Ashton Carter wait for a briefing at the Pentagon on August 4, 2016 in Washington, DC. / AFP PHOTO / Brendan SmialowskiBRENDAN SMIALOWSKI/AFP/Getty Images

(FILES) This file photo taken on November 10, 2015 shows US Secretary of the Air Force Deborah Lee James during a press conference at the Dubai Airshow. The head of the US Air Force on August 4, 2016 said she would be "concerned" if President Barack Obama were to formally declare a "no-first-use" policy for America's arsenal of nuclear weapons. US media reports have in recent weeks said Obama is weighing an overhaul of long-standing nuclear policy, including by pledging to never conduct the first strike in a nuclear conflict."I would be concerned about such a policy," Air Force Secretary Deborah Lee James told the New America think tank in Washington. / AFP PHOTO / MARWAN NAAMANIMARWAN NAAMANI/AFP/Getty Images

U.S. President Barack Obama speaks at a Young African Leaders Initiative town hall in Washington, U.S., August 3, 2016. REUTERS/Yuri Gripas - RTSKXV6

Music executives are sworn in to testify about music licensing before a hearing of the U.S. House Judiciary Committee on Capitol Hill in Washington June 25, 2014. Seen here are singer and songwriter Rosanne Cash (L-R), Recording Industry Association of America (RIAA) Chairman and CEO Cary Sherman, YMF Media Senior Advisor Charles Warfield, Secretly Group Co-founder Darius Van Arman, Radio Music License Committee Inc Chairman Ed Christian, American Society of Composers Authors and Publishers (ASCAP) President and Chairman Paul Williams, Pandora Media Inc Vice President Chris Harrison, SoundExchange Inc President and CEO Michael Huppe and Sirius XM CFO David Frear. REUTERS/Jonathan Ernst

U.S. President Barack Obama answers a question as he and Singapore's Prime Minister Lee Hsien Loong hold a joint news conference at the White House in Washington, U.S. August 2, 2016. REUTERS/Jonathan Ernst

TOPSHOT - US President Barack Obama speaks at the Presidential Summit of the Mandela Washington Fellowship for Young African Leaders at the Omni Shoreham Hotel August 3, 2016 in Washington, DC. / AFP PHOTO / Brendan SmialowskiBRENDAN SMIALOWSKI/AFP/Getty Images

TOPSHOT - US President Barack Obama speaks at the Presidential Summit of the Mandela Washington Fellowship for Young African Leaders at the Omni Shoreham Hotel August 3, 2016 in Washington, DC. / AFP PHOTO / Brendan SmialowskiBRENDAN SMIALOWSKI/AFP/Getty Images