South Korea's new centre-left leader also warned that "rising protectionism and supply chain restructuring" posed 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 snap elections, 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 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 the country 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 Yoon, as he did not immediately attach preconditions to dialogue with Pyongyang, 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.

"While it remains to be seen how Pyongyang will respond, it is notable that Lee has clearly indicated a different approach to North Korea," he added.

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

Just a few hundred guests attended, in contrast to the large outdoor events held after regularly scheduled elections, which often attract tens of thousands.

He will later head to the presidential office to begin naming key cabinet members, with the focus on who is tapped to serve as presidential chief of staff, prime minister and director of the National Intelligence Service.

The 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".

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.