South Korea's new President Yoon Suk-yeol called for the "complete denuclearisation" of the North at his swearing-in on Tuesday, describing Pyongyang's weapons as a threat to regional and global security.

Yoon, 61, who started work in an underground bunker with a security briefing on North Korea, took office at a time of high tensions on the peninsula, with Pyongyang conducting a record 15 weapons tests since January, including two launches last week.

In his inaugural speech, he said he would consider sending significant economic aid to the North, but only if Pyongyang first gives up its nuclear programme -- a demand that is anathema to Kim Jong Un and will likely stymie any chances of dialogue, according to analysts.

"If North Korea genuinely embarks on a process to complete denuclearisation, we (will) present an audacious plan that will vastly strengthen North Korea's economy and improve the quality of life for its people," Yoon said.

Kim's missiles and nukes were a threat to South Korean, regional and global security, he said, adding: "The door to dialogue will remain open so that we can peacefully resolve this threat."

Yoon's conservative administration looks set to usher in a more muscular foreign policy for the world's 10th-largest economy, after the dovish approach pursued by his predecessor Moon Jae-in during his five years in office.

Under Moon, Seoul pursued a policy of engagement with Pyongyang, brokering summits between Kim and then-US president Donald Trump. But talks collapsed in 2019 and diplomacy has stalled since.

Yoon is not likely to have an easy ride, taking office with some of the lowest approval ratings -- around 41 percent, according to a recent Gallup poll -- of any democratically elected South Korean president.

He has relocated the presidential office from the decades-old Blue House, which soured public sentiment as many view the costly move as unnecessary.

Yoon said the Blue House, located at a site used by the Japanese colonial administration from 1910 to 1945, was a "symbol of imperial power", claiming the relocation would ensure a more democratic presidency.

The Blue House grounds will be opened to the public as a park, and during the inauguration ceremony, footage of people walking up to the once-fortified compound was broadcast live.

'Never succumbed'

The formal inauguration ceremony was staged outside Seoul's National Assembly, featuring marching army bands, soldiers in ceremony dress, and a 21-gun salute.

During his inauguration speech, Yoon said that South Korea was facing "multiple crises," citing the pandemic, global supply chain issues and economic woes, and "complex" armed conflicts and wars.

"Such complex, multi-faceted crises are casting a long and dark shadow over us," he said, adding that he was confident the country would emerge from its current difficulties.

"Koreans never succumbed; we became stronger and wiser," he said.

Around 40,000 people attended the massive inauguration ceremony, which local reports said was the country's the most expensive such event by far, at 3.3 billion won ($2.6 million).

Moon and impeached former president Park Geun-hye -- recently pardoned and released from jail by Moon -- both attended the ceremony.

US President Joe Biden designated Douglas Emhoff, husband of US Vice President Kamala Harris, to lead an eight-member delegation.

Japan and China also sent high-level representatives, with Yoon saying he wants to mend sometimes fractious relations with regional powers.