Singapore's state prosecution arm has demanded an apology from several websites over posts which it said cast doubt on the judiciary's integrity in a case involving a China national.

The websites and Facebook pages involved had suggested that a Singapore court had been lenient to Yuan Zhenghua, 31, who hijacked a taxi last year and crashed it into the driveway of Changi Airport's budget terminal, killing a Malaysian airport worker.

Yuan, a technician, was sentenced by a district court to 25 months in jail Monday.

Singaporeans took to Internet websites and Facebook pages to criticise the verdict, prompting the Attorney-General's Chambers (AGC) to issue letters asking for an apology and that the postings be taken down.

A spokesman for the AGC told AFP the online comments and postings had accused the court of bias in favour of the China national.

"These comments include allegations that the courts did not wish to offend the People's Republic of China, that Singaporeans would get heavier sentences in comparison, that the judge lacked integrity, and that leniency was shown," the spokesman said.

"These and similar comments pose a real risk that public confidence in the administration of justice would be undermined."

The spokesman added that the online comments ignored the fact that the court took into account Yuan's "psychotic disorder" in its decision.

While the AGC did not identify the websites involved, the administrators of Facebook page "EDMW loves Singapore" put up an apology for committing an "act of contempt of court" in its posts about Yuan and said it had taken them down.

Some bloggers voiced dismay with the apology demand, citing it as an example of an attempt by the government to clamp down on rising online dissent.

"Our public institutions and public officials should accept and allow a wider threshold for criticisms from the public, including those online," Andrew Loh, a socio-political blogger, told AFP.

The government says attacks on the judiciary will undermine public confidence in the institution.

In July last year, the AGC also sent a letter to prominent Singaporean blogger Alex Au after he alleged in a post that the city-state's courts are biased in favour of the well-connected.

In 2010, a Singapore court jailed British author Alan Shadrake for six weeks for publishing a book critical of the city-state's administration of the death penalty.