The sources added the letter also expressed Ban's gratitude for greeting him during his visit to Korea in May, and asked Kim to give him "guidance" in the future as well.

Kim, a former prime minister still has symbolic importance even though he has retired from frontline politics.

Political pundits said the letter may reflect Ban's wish to seek the country's highest elected office. They added that it could also spark renewed debate, particularly among conservatives on the need to rally behind Ban next year, since there is a scarcity of contenders who could take on liberal candidates in the 2017 election.

During his visit to South Korea earlier in the year, Ban said he would "contemplate" what he would do as a South Korean citizen when he returns to his home country after completing two terms as the U.N. helmsman at the end of this year.

While Ban never clarified his plan, he has been standing at the top of the public survey on preferred potential candidates.

According to the survey compiled by local pollster RealMeter on Thursday, Ban's approval rate came to 38 percent, handily beating Moon Jae-in, the former head of the Minjoo Party of Korea whose numbers stood at 31.8 percent.

A source in Korea's foreign ministry, meanwhile, said that while some media outlets have raised issue with the possibility that Ban's letter may have been sent through a diplomatic pouch, he said there is nothing unusual if it did. He pointed out that using diplomatic pouches to deliver certain letters is a standing diplomatic tradition. (Yonhap)