Fighter jets flew over downtown Seoul and tanks rolled through the streets, as South Korea displayed weaponry including its largest ballistic missile, the Hyunmoo-5, which is capable of destroying underground bunkers, for the first time.

The event, which marks Armed Forces Day in the South, comes as relations between the two Koreas are at one of their lowest points in decades, with Pyongyang declaring the South its "principal enemy" and moving nuclear-capable weapons to the border.

"If North Korea attempts to use nuclear weapons, it will face the resolute and overwhelming response of our military and the US and Republic of Korea alliance," Seoul's President Yoon Suk Yeol said.

"That day will be the end of the North Korean regime," Yoon said, addressing thousands of service members gathered at Seoul Air Base for the event.

Washington stations tens of thousands of troops in South Korea to help defend it against the North, and Seoul -- which has no nukes of its own -- is covered by the US nuclear umbrella.

Yoon urged the North to "abandon the delusion that nuclear weapons will protect them", and slammed Pyongyang's recent campaign of floating trash-carrying balloons into the South, which have caused airport disruptions and triggered small fires.

- Nuclear umbrella -

An American B-1B heavy bomber staged a flyover of the ceremony early Tuesday, flanked by F-15K jets.

Washington periodically deploys nuclear assets to the Korean peninsula, as part of the nuclear umbrella to protect the South from Pyongyang's growing threats.

North Korea slammed the B-1B flight and warned it would take "corresponding" action against what it deemed America's "reckless military bluff," according to a statement carried by the official Korean Central News Agency.

The North Korean military would be "keenly watching the frequent deployment" of US strategic assets to the peninsula, said Kim Kang Il, the vice-minister of national defence.

Pyongyang's military is "fully prepared to thoroughly defend" the nuclear-armed state, he added.

Yoon's warning comes weeks after the North disclosed images of a uranium enrichment facility for the first time, showing leader Kim Jong Un touring the site as he called for more centrifuges to boost the country's nuclear arsenal.

South Korea's spy agency later stated that the unprecedented disclosure was "directed at the US" and that North Korea was believed to be capable of producing a double-digit number of nuclear weapons from its stockpile of highly enriched uranium and plutonium.

Last week, a lawmaker told reporters that the National Intelligence Service had warned the North might carry out another nuclear test -- its seventh -- after the US elections in November.

- Target Kim's bunker? -

"The public display of the Hyunmoo-5 is aimed at the North," Hwan Kwon-hee of Korea Association of Defense Industry Studies told AFP.

The move signals that "while Seoul doesn't have nuclear weapons, it possesses a sufficient stockpile of conventional weapons capable of reducing Pyongyang to rubble when fired in large numbers during a full-scale war," he said.

With a weapon on the scale of the Hyunmoo-5 -- which experts say can carry an eight-ton warhead -- this could include "targeting underground bunkers where Kim Jong Un is likely to hide," he added.

Thousands of people attended the military parade, which was being held for a second consecutive year in Seoul's central Gwanghwamun square, involving 3,000 personnel and 80 pieces of equipment.

The last time Seoul held consecutive military street parades for Armed Forces Day was in 1984, under the dictatorial rule of the late Chun Doo-hwan.