Singapore Prime Minister Lee Hsien Loong said Wednesday he had ordered the transport minister to go on leave after the city-state's powerful anti-graft body launched a corruption probe.

Lee said the Corrupt Practices Investigation Bureau (CPIB) had briefed him about a case it had uncovered and sought his approval for a formal investigation that would involve interviewing Transport Minister S. Iswaran, among other people.

"I have instructed Minister Iswaran to take leave of absence until these investigations are completed," Lee said in a Facebook post.

The CPIB said Iswaran "is currently assisting" with its investigation of a case it had uncovered.

It said it could not disclose details as the investigation was still underway.

"CPIB acknowledges the interest by members of the public in this case because a minister is being interviewed... CPIB will investigate this case thoroughly with strong resolve to establish the facts and the truth, and to uphold the rule of law," it said in a statement.

"Singapore adopts a strict zero-tolerance approach towards corruption. CPIB investigates all cases without fear or favour and will not hesitate to take action against any parties involved in corrupt activities."

Regional financial hub Singapore has been consistently ranked in surveys as among the least corrupt countries in the world.

Salaries of ministers are benchmarked against top wages in the private sector, with founding prime minister Lee Kuan Yew saying this helps attract the best people to join the government and deter corruption.

Singapore's prime minister earns around Sg$2.2 million ($1.6 million) a year including bonuses, four times the salary of the US president.

Ministers earn around Sg$935,000, including variable bonuses, according to the government's Public Service Division.