FORMER UN secretary-general Ban Ki-moon said he will soon announce whether he will enter the race to be South Korea’s president, as he returned home to be greeted by cheering supporters.

Ban is seen as the only major conservative contender if there is an early election to replace impeached President Park.

Dozens of politicians have already bolted from Park’s conservative ruling party and want to align with Ban, while the main liberal opposition party is escalating the political offensive against him.

"Many people have asked me whether I have a will for power," Ban told a televised conference upon his arrival at Incheon International Airport, near Seoul.

"If 'will for power' means uniting a divided country into one and making it a first-class country again, then I have already said I am ready to burn my body in devotion to this and my mind hasn't changed," he said.

As his supporters shouted his name, Ban, a former South Korean foreign minister, said he will get diverse opinions about his return to South Korean politics.

"Based on my talks with people, I will make a decision that will be free from selfish motives. That decision won't take long," he said.

Opinion polls show Ban is one of the favourites to succeed Park, who is now on trial at the Constitutional Court, which will determine whether to formally end her rule and hold a new election.

The opposition-controlled parliament impeached her last month over her alleged roles in an explosive corruption scandal.

South Korea is scheduled to hold a presidential election in December.

Ban, who ended his 10-year service as UN chief last month, is a soft-spoken career diplomat known for a gentle image and an ability to avoid making enemies.

But he also faced criticism that he lacks charisma and experience in domestic politics.

Many South Koreans have taken great pride in him because they think Ban's top UN job represents their country's rise in the international arena from the rubble of the 1950-53 Korean War.

But critics say Ban would not have assumed the UN post without help from the government of late president Roh Moo-hyun, which reportedly made massive efforts to make him a first South Korean UN chief.

Ban denied an allegation in the media that he took bribes from a businessman who was at the centre of a corruption scandal that led to Roh's suicide in 2009.

Two relatives of Ban were charged in the US in an indictment unsealed on Tuesday with plotting to bribe a Middle East official to influence the 800 million dollar sale of a building complex in Vietnam.

Ban's main potential rival is Moon Jae-in, a former leader of the largest opposition Democratic Party who lost the 2012 election to Park.

A survey released this week by Realmeter showed Moon had a 27.9 per cent approval rating compared with Ban's 20.3 per cent. The survey of 1,511 respondents had a margin of error of 2.5 points.