Former UN chief Ban Ki-moon, once considered to be a prospective presidential candidate in his home country of South Korea, has ruled out a run for the job, saying he was "disappointed at the selfish ways" of some politicians.

Ban told reporters on Wednesday at parliament, after meeting conservative party leaders, that he had been subject to "malign slander akin to character assassination" in the media and had given up his "patriotic" plan to lead political change.

It would be "pointless to find a common path with them", he said.

Ban arrives in South Korea and hints at presidency bid

While Ban never officially confirmed his intention to run, he hinted in several interviews that he wanted to work towards unifying the country that has been divided over a corruption scandal surrounding President Park Geun-hye.

"With all kinds of fake news, my intention for political change was nowhere to be seen and all that was left was grave scars to my family and myself, and to the honour of the UN, where I spent the past 10 years," he said.

South Korea has been gripped by political crisis for months amid a corruption scandal that led to the impeachment of the president. If the impeachment vote is upheld by the Constitutional Court, she will have to quit and an election will be held two months later.

A ruling is expected as soon as late this month.

Ban, 72, returned to South Korea on January 12 after serving 10 years as UN secretary-general. He was unable to capitalise on his much-anticipated homecoming, mired in a series of perceived PR gaffes and a scandal involving family members.

The media leapt on a series of minor blunders, for instance when he took the airport express train instead of a limo on his return to South Korea, but did not know how to buy a ticket.

Support ratings fall

Two days later, Ban visited a care home where he fed porridge to an old woman. He was criticised for wearing a bib when the old woman was not - and for feeding someone lying flat on their back.

Even without announcing his intention to run, his support ratings in opinion polls had slipped to second place behind the presidential candidate for the main opposition Democratic Party, Moon Jae-in, after peaking at nearly 30 percent last year.

Ban had been expected to run as a conservative but was unable to secure any party affiliation.

Ban's clean image and his international profile were dealt a blow with the indictment of his brother, Ban Ki-sang, and a nephew in the United States in a bribery scheme involving a Vietnamese development project.

Ban's announcement appeared to take the four main political parties aiming to field candidates by surprise, including Moon's Democratic Party.

"I was looking forward to a good race, so it is disappointing," he told reporters.

A poll by R&Search released on Wednesday showed Ban's support continuing to slip to 16.5 percent from 18 percent a week ago, compared with 35.2 percent for Moon, up from 34.8 percent a week ago.

Ban's decision could boost the chances of minor candidates such as Ahn Cheol-soo of the progressive People's Party, Kim Jun-seok, a political science professor at Dongguk University in Seoul, told Reuters.

Ban was South Korea's foreign minister from 2004 to 2006, helping to implement a policy of engagement with North Korea before taking the top job at the UN.