Tensions on the Korean peninsula have risen markedly in recent months after North Korea's latest missile and nuclear tests, conducted in defiance of worldwide pressure and United Nations resolutions.
The South Korean army's adoption of drones on the battlefield follows the footsteps of the USA and Chinese armed forces, which have already adopted swarming drone tactics.
The long-range, multi-mission supersonic aircraft would conduct bombing drills with South Korean fighter jets, Yonhap said earlier.
The U.S. Seventh Air Force has said the exercise is not in response to any incident or provocation.
It is increasingly common for the U.S. to deploy B-1B bombers over South Korea to underline military superiority against North Korea. Lindsey Graham (R-SC) argued Sunday that the USA should stop sending military dependents to South Korea and transfer those that are already there.
Feltman, a former senior U.S. State Department official, is the highest-level United Nations official to visit North Korea since 2012.
North Korea hates such displays of American military might at close range and typically uses strong language to condemn them as invasion rehearsals.
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US Presidential Donald Trump has repeatedly stated that China is key to unlocking the North Korea issue.
The US and South Korea kicked off the five-day war games - called Vigilant Ace - on Monday, with more than 200 aircraft, which have been conducting mock attacks on North Korea in different war scenarios.
North Korea regularly cites military drills around the peninsula as justification for its nuclear and missile-testing program.
All but five of the 49 countries have violated sanctions in other ways, such as facilitating front companies for the North Korean regime or importing sanctioned goods and minerals.
Paintings from North Korea's most celebrated artists are being sold at high prices, with some works priced in the tens of thousands of dollars, according to the report. China is North Korea's lone major ally and fears widespread instability on its border.
Tensions have been running high since the missile launch late last month, which triggered global condemnation and a warning from the USA that the North Korean regime would be "utterly destroyed" if war broke out.