The frontrunner to be South Korea's next president has apologised after sparking a public uproar for jokingly likening the skin colour of an African student to coal.

Kim Moo-Sung, the leader of the country's ruling conservative Saenuri Party and the leader in polling ahead of December 2017 elections, issued the apology in the face of public criticism over the gaffe.

"I failed to realise that showing friendliness this way could hurt someone's feelings," Kim wrote on his Facebook page on Friday.

"I am really sorry and there is no excuse. I sincerely apologise from the bottom of my heart."

He made the quip while delivering coal briquettes to the needy in Seoul alongside a group of foreign exchange students earlier on Friday.

The event was part of a charity programme which drew around 40 students from 27 countries as well as 50 young members of the Saenuri Party.

"Your face colour is the same as the briquettes' colour," the 64-year-old said to a Nigerian student while laughing.

The student, Babalola Joushua Adekunie, said he did not take offence.

"I don't give any special meaning to it," Adekunie told local news outlet Focus News. "He said it jokingly. He was laughing."

"Personally I'm a black man... It's nothing that I am ashamed of."

Asked whether he thought Kim should apologise to him, Adekunie replied in Korean: "Guenchanayo (No problem)".

The 26-year-old student is attending a local university on a South Korean government scholarship.

Kim topped a Real Meter survey published last week on support levels for various presidential hopefuls with 22.5 percent of respondents backing him. Moon Jae-In of the main opposition party was second on 16.1 percent. (AFP)