Yoon is on a week-long US state visit to mark the 70th anniversary of the friendship, amid saber-rattling from the North, the war in Ukraine and other global challenges facing both countries.

"Our alliance was forged 70 years ago to defend Korea's freedom. The alliance has now become a global alliance to safeguard freedom and peace around the world," he said.

Echoing President Abraham Lincoln's Gettysburg Address, he greeted the lawmakers by hailing a "new nation, conceived in liberty, and dedicated to the proposition that all men are created equal."

"We have many reasons to celebrate our platinum anniversary. We had no guarantees of success when he started but today our alliance is stronger than ever," he added.

Congressional leaders invited Yoon for the speech in a letter underscoring the "great importance and significance" of the relationship between the allies.

He is the third foreign leader to address a joint meeting of Congress since President Joe Biden took office -- after appearances last year by Greek Prime Minister Kyriakos Mitsotakis and Ukrainian President Volodymyr Zelensky.

Yoon met Biden on Wednesday at the White House, where the allies warned North Korea it would face a nuclear response and the "end" of the leadership there if Pyongyang uses its own arsenal.

Yoon and Biden issued what was titled the Washington Declaration, bolstering the US nuclear umbrella over South Korea, which is increasingly nervous about the Pyongyang's aggression.

- 'Korea will never forget' -

"We need to speed up Korea, US, Japan trilateral security cooperation to counter increasing North Korean nuclear threat," Yoon warned Congress, vowing to work for Pyongyang's denuclearization.

"We must raise global awareness of (the) dire human rights situation in North Korea. We must not shy away from our duty to promote freedom for North Koreans," he added.

Yoon and Biden had rounded off a day of pomp and ceremony Wednesday with a lavish state dinner attended by Hollywood star Angelina Jolie -- with Yoon surprising guests with a rendition of his favorite song, "American Pie."

Biden had recalled the sacrifice by American soldiers to help fight the communist north during the 1950-1953 Korean War.

"Korea will never forget the great American heroes who fought with us to defend freedom," Yoon told Congress on Thursday.

Turning to modern conflict, he condemned Russia's invasion of Ukraine as a "violation of international law," pledging to work for the freedom of the Ukrainian people and to support the country's reconstruction.

There were conspicuous gaps across the floor for what could have been an awkward affair, seven months after Yoon was overheard calling US lawmakers who rejected funding for a global health program "idiots."

But he was warmly received by the majority of members who did show up, receiving a raucous ovation on his entry into the chamber for an address that was punctuated by exuberant applause.

He prompted laughter across the chamber as he joked that K-Pop band BTS, who visited Biden earlier this year, "beat him to the White House, but I beat them to Capitol Hill."

Yoon was due to have lunch with Vice President Kamala Harris and Secretary of State Antony Blinken later Thursday before a visit Friday to MIT and Harvard University in Boston and his return home on Saturday.