President Obama met and shook hands with South Korean Internet sensation Psy at a holiday concert in Washington Sunday night, amid the controversy surrounding the rapper's 2004 anti-American comments calling for the killing of "Yankees who have been torturing Iraqi captives."

Psy, whose hit, "Gangnam Style," has become the most-watched YouTube video of all time, apologized Friday for a 2004 performance in which he said U.S. soldiers fighting in the Iraq War should be killed "slowly and painfully."

His song came on the heels of the decapitation of a Korean missionary by Islamic militants in Iraq.

"Kill their daughters, mothers, daughters-in-law, and fathers. Kill them all slowly and painfully," he sang at the concert in Seoul.

He has apologized for the lyrics. "I understand the sacrifices American servicemen and women have made to protect freedom and democracy in my country and around the world," Psy said in a statement Friday.

He said the song "was part of a deeply emotional reaction to the war in Iraq and the killing of two Korean schoolgirls that was part of the overall anti-war sentiment shared by others around the world at that time."

President Barack Obama shakes hands with South Korean musician PSY, center, with Conan O'Brien and performer Scotty McCreery, during the "Christmas in Washington" concert at the National Building Museum on Dec. 9, 2012, in Washington. (Molly Riley-Pool/Getty Images)

Obama, in brief remarks at the "Christmas in Washington" concert Sunday, gave a shout-out to several of the artists who performed, not including the Korean pop-singer, 34.

"Tonight is about Conan and Diana Ross and Santa and all the other talented folks on this stage," Obama said. "But it's also about the Americans who are spending this holiday in a hospital bed, or a shelter, or protecting our freedom on a battlefield far from home.

"Let's keep them in our prayers, and follow Christ's calling to love one another as He has loved all of us."

The annual charity function benefits the National Children's Medical Center and the entertainment is not chosen by the White House.

- Mary Bruce and Jake Tapper

ABC News' Sheila Marikar contributed to this story.