Yoon, 61, takes office at a time of high tensions on the Korean peninsula, with an increasingly belligerent Pyongyang conducting a record 15 weapons tests since January, including two launches last week.

His 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 outgoing President Moon Jae-in during his five years in office.

At midnight Tuesday, Yoon began his five-year term with his first briefing as commander-in-chief from the Joint Chiefs of Staff at an underground bunker set up at his new presidential office.

Yoon stressed the need to maintain a "firm military readiness posture at a time when the security situation on the Korean peninsula is critical," his office said in a statement.

After winning the election in March by the narrowest margin, he vowed to "sternly deal" with the threat posed by Kim Jong Un's regime, while saying he would leave the door to dialogue open.

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.

Moon remains personally popular, but public frustration with his administration helped sweep Yoon to power.

But it is not likely to be an easy ride: Yoon is 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, was a "symbol of imperial power", claiming the relocation will ensure a more democratic presidency.

It will be opened to the public as a park.

- Expensive ceremony -

A bell-ringing ceremony was held at the stroke of midnight in downtown Seoul, but Yoon's formal inauguration ceremony will be held later Tuesday at the National Assembly.

Yoon's inauguration speech will be closely parsed for any indication of how he intends to begin his five-year term.

Local reports say Yoon is writing the speech himself and that it will focus on three keywords: freedom, market and fairness.

Around 40,000 people have been invited to attend the inauguration ceremony, which is by far the most expensive event of its kind at 3.3 billion won ($2.6 million).

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

US President Joe Biden designated Douglas Emhoff, husband of US Vice President Kamala Harris, to lead an eight-member delegation, the White House said in a statement last week.

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