It was the latest move by the Communist state to bolster its claims in the strategic waterway that China claims as its own.
Vietnam's relations with the U.S. have quickly expanded in the wake of the escalating South China Sea dispute, putting the Philippines in an unexpectedly isolated position.
Some think China is preparing an "air defense identification zone" over the South China Sea.
Obama will become the first sitting president to visit the small nation when he joins global leaders for an Asian summit.
The British MP was pictured handling the weapon while enjoying his summer holiday.
China's response to the U.N. verdict will determine what kind of power it will be.
If the ruling goes against China and Beijing responds with a show of force, it will be a critical test of how far the U.S. will go to support its allies.
Officials debate whether the president should roll back an arms embargo on Hanoi, which would anger China.
Security experts have warned this could be the first of many military flights.
Tens of thousands of tourists have also been stranded, as snow grounds flights.
Nguyen Van Dai, who was badly beaten this month, was arrested last week for anti-state "propaganda.
The U.S.'s goal is to remind China and other countries about America's right to freely navigate the region.
Kim Phuc was photographed as she ran away from a napalm attack during the Vietnam War.
A senior U.S. official reportedly said U.S. ships would sail within 12-nautical-mile zones that China claims as territory.