South Korea's new centre-left leader also warned that "rising protectionism and supply chain restructuring" pose an existential threat to Asia's export-dependent fourth-largest economy, which has been buffeted by the global trade chaos sparked by US President Donald Trump.

Lee scored a thumping victory over conservative Kim Moon-soo of disgraced ex-president Yoon Suk Yeol's former party in the snap poll, with his term beginning immediately after the vote tally was certified early Wednesday.

Lee secured 49.4 percent of the vote, far ahead of the 41.2 percent for Kim -- who conceded, having been hampered by party infighting and a third-party candidate splitting the right-wing vote.

Lee spoke to South Korea's top military commander and formally assumed operational control of the country's armed forces Wednesday, urging them to maintain "readiness" in case of Pyongyang's provocations -- but said in his first comments that he was ready to talk.

"We will heal the wounds of division and war and establish a future of peace and prosperity," he said.

"No matter how costly, peace is better than war."

He said Seoul would "deter North Korean nuclear and military provocations while opening communication channels and pursuing dialogue and cooperation to build peace on the Korean Peninsula".

Lee took office just hours before the United States was set to slap tariffs of 50 percent on South Korea's crucial steel and aluminium exports.

"The rapid changes in the global order such as rising protectionism and supply chain restructuring pose a threat to our very survival," Lee said.

Markets reacted favourably to the election, with the benchmark KOSPI and the won rising Wednesday.

- 'Significant departure' -

Lee's comments on North Korea are a "significant departure" from those of his hawkish predecessor as he did not immediately attach preconditions to dialogue, said Hong Min, a senior analyst at the Korea Institute for National Unification.

It signals "his desire to resolve disagreements through talks", Hong told AFP.

Lee held a modest inauguration ceremony at the National Assembly -- where Yoon deployed armed troops on the night he attempted to suspend civilian rule.

He also announced a number of top appointments, including long-time adviser Kim Min-seok as prime minister and former unification minister Lee Jong-seok as spy chief.

Lee's day is expected to end with a flurry of congratulatory phone calls from world leaders, with Trump likely to be the first on the line.

Trump's top diplomat, Marco Rubio, was swift to offer his own congratulations and voice hope for working with Lee, who previously has sought greater distance from the United States.

Washington's alliance with Seoul was "ironclad", the US secretary of state said in a statement, citing "shared values and deep economic ties".

In a statement to Seoul's Yonhap news agency, the White House described the election as "free and fair".

But it added: "The United States remains concerned and opposed to Chinese interference and influence in democracies around the world".

Unsubstantiated conspiracy theories have swirled in South Korea in recent months about alleged Chinese interference in the country's politics.

Japanese Prime Minister Shigeru Ishiba also said he wanted to "energise cooperation" between Seoul and Tokyo, South Korea's former colonial master.

And India's Narendra Modi said on X he wanted to "strengthen" ties with Seoul as he congratulated Lee.

- 'Positive direction' -

Lee comes to power with his party already holding a parliamentary majority -- secure for the next three years -- meaning he is likely to be able to get his legislative agenda done.

On the streets of Seoul, South Koreans said they welcomed Lee's overtures to the North.

"Since our economy and many other aspects of society are closely linked to the state of inter-Korean relations, I hope we can take a long-term perspective and move in a more positive direction," Choi Ki-ho, 55, told AFP.

Lee Ju-yeon, a 42-year-old quasi-public sector employee, said they hoped Lee "will devote himself to uniting our divided nation".