Singapore's transport minister has been arrested in connection with a rare top-level corruption probe that has also ensnared a billionaire hotel tycoon, the country's anti-graft body has said.

Transport Minister S. Iswaran was "arrested on 11 July 2023" and "subsequently released on bail", the Corrupt Practices Investigation Bureau (CPIB) said in an emailed statement late Friday, confirming the arrest for the first time.

Hotel tycoon Ong Beng Seng, one of Singapore's richest people, was also arrested on the same day and released on bail in connection with the probe, the bureau said.

The bureau has not disclosed details of the investigation that has gripped the city-state, a global financial hub reputed to be among the least corrupt countries in the world.

Cabinet ministers are paid salaries comparable to the top earners in the private sector to deter corruption.

The investigation of the transport chief by the powerful anti-graft bureau widened after it was disclosed that Ong was included in the probe.

Ong is the managing director of Hotel Properties Limited (HPL), which owns a string of high-end hotels and resorts in locations around Asia and the Pacific.

Ong Beng Seng, one of Singapore's richest men, is credited with bringing Formula 1 to the country. Photo: AFP

In a filing with the Singapore Exchange on Friday, the company said that Ong was given a "notice of arrest" by the CPIB and had posted bail.

While the passports of those under arrest are generally impounded, Ong was granted permission to leave Singapore on Friday, with the CPIB noting it considers requests for travel overseas "on a case-by-case basis".

The CPIB said it had "acceded to Ong's request to travel overseas", but increased his bail to Sg$100,000 ($76,000).

"Upon his return, Ong is required to report to CPIB and surrender his passport to the bureau," the CPIB added.

Ong, a Malaysian who is a Singapore permanent resident, is credited with helping bring the Formula One Grand Prix to Singapore in 2008.

His private company Singapore GP and the Singapore Tourism Board last year renewed the contract to host the F1 race until 2028.

Iswaran, meanwhile, had been ordered to go on leave earlier in the week by Prime Minister Lee Hsien Loong after the CPIB said the minister was "currently assisting" an ongoing investigation.

Lee said the anti-graft body had sought his approval for a formal investigation that would involve interviewing Iswaran, among other people.