Major Mykola Melnychenko fled Kiev in 2000 after revealing excerpts from secret recordings he made of Mr Kuchma which implicated the president in the murder of a journalist who had exposed the corruption of Mr Kuchma's 10-year rule.
Major Melnychenko has been provided with a bodyguard by the Ukrainian intelligence services since his arrival on Wednesday from the US where he was given political asylum in 2003. He was with a former Ukrainian MP, Oleksandr Yalyaskevych, also granted US asylum after claiming Mr Kuchma's henchmen had tried to kill him.
Major Melnychenko said: "We will do everything we can to see that Kuchma's gang, which carried out great crimes, should be held responsible for everything they did."
The headless corpse of the journalist Georgiy Gongadze was discovered in November 2000. The murder became a rallying cause for the orange revolution a year ago that ousted the Kuchma regime.
Major Melnychenko says he saw Mr Kuchma taking bribes, and recorded him allegedly authorising sales of sophisticated defence systems to Iraq in contravention of UN sanctions.
Mr Kuchma has denied wrongdoing and says the recordings are doctored. Independent experts concluded they are authentic.
President Viktor Yush-chenko, who led the orange revolution, has lost popularity because none of the politicians from Mr Kuchma down, and oligarch businessmen he branded criminals, have been charged.
Source: The Independent