"The tension between the South and the North is at its peak, and so it is a very cautious situation as if we are walking on thin ice", Moon said, according to South Korea's Yonhap news agency.
The Chinese air force launched an exercise of their own concurrent to the enormous air drills being run by the US and South Korea, causing experts to speculate what, exactly, Beijing is trying to communicate to the West.
"Our personnel forward deployed to South Korea remain in a consistent state of readiness through various training exercises", Benham added - including drills that prepare families for a possible noncombatant evacuation operation, Stars and Stripes reported.
"The largest joint air force exercise ever can be seen as a way of putting pressure on North Korea and showing a military demonstration to stop any further actions from North Korea, such as a seventh nuclear test or ICBM tests", Kim Yong-Hyun of the Dongguk University Graduate School said. The U.S. Air Force spokesman could not confirm the reports.
US stealth fighter jets were not mobilized in Thursday's exercise, the military said, which saw the two strategic bombers enter the Korea Air Defense Identification Zone (KADIZ), fly to Jeju island and up the west coast, before returning to Andersen Air Force Base.
The South Korean army is reportedly on course to launch a combat unit consisting of drone bots sometime next year, in a move that signals a new era of drone bots in the military.
During the Korean War that began in 1950 and is still technically ongoing, China came to the aid of the communist Northern Korean factions against the U.S. and their Southern allies, repaying the fledgling North's contribution to their own civil war, which the Chinese Communist Party had only won the previous year with Korean communist help.
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North Korea's state media confirmed the arrival of Feltman and his entourage late on Tuesday without offering more details.
The Guam-based Lancer bombers trained with more than 20 fighter jets as part of the Vigilant Ace joint exercise with South Korea, which began on December 4 and will continue until Friday.
"We hope that all relevant parties could exercise restraint and refrain from heightening tensions on the Korean Peninsula and provoking each other".
Feltman, a former senior U.S. State Department official, is the highest-level United Nations official to visit North Korea since 2012. The drills follow a successful test by the DPRK of an intercontinental ballistic missile most analysts believe could reach the mainland United States.
The joint exercise is created to enhance readiness and operational capability and to ensure peace and security on the Korean peninsula, the USA military had said before the drills began.
Russia also has communication channels open with North Korea and is ready to exert its influence, the RIA news agency quoted Russian Deputy Foreign Minister Igor Morgulov as saying on Tuesday.
It says its weapons programmes are a necessary defence against USA plans to invade.