North Korea against ongoing war games.The deployment of the stealth bombers was intended to send a potent message to Pyongyang about the US commitment to defending South Korea against any aggression, as military tensions on the Korean peninsula soar.
It came shortly after the North severed its last-remaining military hotline with South Korea and put its rocket units on combat status with a threat to target US bases in the Pacific region.
The two B-2s, from Whiteman Air Force Base in Missouri, flew the 13,000-mile (20,800-kilometre) round-trip in a "single continuous mission", dropping dummy ordnance on a target range in the South, the US military said in a statement.
"This demonstrates the United States' ability to conduct long-range, precision strikes quickly and at will," the statement said.
The bombers were participating in South Korean-US military exercises that have incensed North Korea, which has threatened to unleash a second Korean War and launch pre-emptive nuclear strikes on South Korea and the US mainland.
"The United States is steadfast in its alliance commitment to the defense of the Republic of Korea," the US statement said.
Earlier Thursday, US Defense Secretary Chuck Hagel told his South Korean counterpart that Seoul could rely on all the military protection the United States has to offer -- nuclear, conventional and missile defence.
The US and South Korean militaries signed a new pact last week, providing for a joint military response to even low-level provocation by North Korea.
Yonhap news agency quoted a Seoul military official as saying Washington had recently approved the sale of US "bunker buster" bombs to South Korea -- capable of targeting the North's extensive underground military facilities.