Former UN Secretary-General Ban Ki-moon left for the United States on Saturday to lecture as a visiting professor at Harvard University for three months.

Speaking to reporters at Incheon International Airport, he expressed concern about public divisions in South Korea that can be seen during the presidential campaign. 

He said his heart is heavy to leave Korea for the U.S. to attend a research program at Harvard, even though he'll only be gone a for a short time. 

Ban also said he was sorry for not living up to the public's support and expectations. 

He also positively assessed serving as a special diplomatic envoy, an idea proposed by People's Party presidential candidate Ahn Cheol-soo.

He said he is willing to play his due part as a South Korean citizen and will accept any call requiring his experience regardless of who becomes president.