LOS ANGELES (Reuters) - The South Korean pop singer behind the viral smash hit "Gangnam Style" apologized on Friday for past concerts featuring anti-American lyrics, ahead of a holiday performance to be attended by U.S. President Barack Obama and his family.

Psy issued the apology after reports surfaced in the United States on Friday about his participation in two performances critical of the U.S. military in 2004.

Psy's "Gangnam Style" Korean pop and dance video is now the most-watched video ever on YouTube, with more than 900 million views since it was first uploaded in July.

"While I'm grateful for the freedom to express one's self, I've learned there are limits to what language is appropriate and I'm deeply sorry for how these lyrics could be interpreted," the rapper said in a statement.

"I will forever be sorry for any pain I have caused by those words," he added.

In one performance, which Psy said was from eight years ago, the rapper protested the deaths of two teenage South Korean girls who were run over by a U.S. tank stationed in the country.

In a separate performance, Psy was critical of the U.S.-led invasion of Iraq and its occupation, in which South Korean forces participated.

Psy is scheduled to perform at the annual "Christmas in Washington" television special that will also be attended by Obama and his family, the White House said on Friday. Broadcaster TNT said Psy would still perform as planned.