The US has flown two B-2 stealth bombers over South Korea as part of a military exercise.
The US said it demonstrated its forces could conduct "long-range, precision strikes quickly and at will".
The move follows strong rhetoric from Pyongyang and comes a day after it cut a military hotline with the South.
The hotline had been used mainly to facilitate cross-border travel at a joint industrial complex, which was said to be operating normally.
More than 160 South Korean commuters went through border control on Thursday morning to start work at the Kaesong complex, after being approved for entry by North Korea, officials said.
North Korean authorities had used a civilian phone line to arrange the crossing, they added.
The joint project is a source of badly-needed hard currency for the North. Around 120 South Korean firms operate at Kaesong industrial park, employing an estimated 50,000 North Korean workers.
Pyongyang has been angered both by annual US-South Korea military drills, and the fresh UN sanctions that followed its third nuclear test on 12 February.
The hotline it severed was the last direct official link between the two nations. A Red Cross hotline and another line used to communicate with the UN Command at Panmunjom have already been cut. An inter-Korean air-traffic hotline still exists.
North Korea has also made multiple threats against both the US and South Korea in recent weeks, including warning of a "pre-emptive nuclear strike" on the US and the scrapping of the Korean War armistice.
North Korea is not thought to have the technology to strike the US mainland with either a nuclear weapon or a ballistic missile, but it is capable of targeting some US military bases in Asia with its mid-range missiles.
The US military said in a statement that the B-2 flight showed US "capability to defend the Republic of Korea [South Korea] and to provide extended deterrence to our allies in the Asia-Pacific region".
The two nuclear-capable planes flew from Whitman Air force Base in Missouri to South Korea as part of a "single, continuous" round trip mission during which they dropped "inert munitions on the Jik Do Range", the statement said.
The US said earlier this month that nuclear-capable B-52 bombers were taking part in the annual joint exercises with South Korea, prompting an angry response from Pyongyang.
Meanwhile, in a phone call on Wednesday, US Defense Secretary Chuck Hagel told his South Korean counterpart Kim Kwan-Jin that the US would provide "unwavering" support to South Korea.
He also told his South Korean counterpart that the US-South Korea alliance was "instrumental in maintaining stability on the Korean peninsula", Pentagon spokesman George Little said in a statement.