Singapore has placed the head of its Central Narcotics Bureau under investigation for "serious personal misconduct", the government said Tuesday.

The bureau is the lead agency in the fight against illegal narcotics in Singapore, where drug trafficking is punishable by death.

The probe into Ng Boon Gay began last December, the Ministry of Home Affairs (MHA) said in a statement.

Another official, the head of the civil defence force Peter Lim, is also under investigation, it said.

The ministry did not give details of the allegations, but said Ng and Lim were being investigated by the Corrupt Practices and Investigation Bureau for "serious personal misconduct."

Both officials have been replaced, the government said.

"All public officers, regardless of their position or seniority, are expected to uphold the highest standards of integrity and conduct," Deputy Prime Minister and Home Affairs Minister Teo Chee Hean said in a statement.

"The government takes a serious view of public officers who misconduct themselves. Any allegations of misconduct made against public officers will be investigated fully and thoroughly."

"If officers are found to have abused the trust placed in them, we will not hesitate to take firm and decisive action against them."

The MHA said that "as investigations are ongoing, (the ministry) is unable to comment on the details of the case."

Large-scale graft cases are rare in Singapore, a thriving global financial centre and business hub which enjoys a reputation for being the least corrupt country in Asia.

It pays its civil servants some of the highest government salaries in the world as a deterrent to corruption.

Last year, two senior civil servants were sentenced to 22 years and 15 years in jail, respectively, for their role in one of the worst corruption cases in the country's history.