A court on Monday jailed a 28-year-old Singaporean for two months for defacing the prime minister's office website during a rash of cyber attacks in the city-state last year.

Mohammad Azhar bin Tahir, who is unemployed, had pleaded guilty to "unauthorised modification" of a section of Prime Minister Lee Hsien Loong's official website on November 7, 2013, causing it to display mocking messages and pictures from international hacker group Anonymous.

The hacking was carried out a day before the website of the Istana, the official residence of President Tony Tan, was defaced in a similar manner.

A court spokeswoman told AFP that Mohammad Azhar was sentenced on Monday to two months in jail for defacing the prime minister's office website, and a further four months for other unrelated charges under Singapore's Computer Misuse and Cybersecurity Act.

Court documents said Azhar had injected a computer code into the server of Lee's website, www.pmo.gov.sg, causing a section of it to be compromised.

The hacked section, which was removed in the early hours of November 8, showed the message "ANONYMOUS SG WAS HERE BIATCH".

"It's great to be Singaporean today," read another headline next to Anonymous's trademark Guy Fawkes mask, a symbol of anti-establishment defiance worldwide.

Earlier this year a 43-year-old Singaporean man was fined Sg$8,000 ($6,400) for the Istana incident, while his 18-year-old accomplice was ordered to have his movements restricted for a year.

Separately, Singaporean James Raj Arokiasamy, 35, is facing 162 criminal charges for various computer misuse offences, including illegally accessing the parliamentary district website of premier Lee and a reporter's blog on The Straits Times's website.

The rash of attacks late last year took place after a self-proclaimed spokesperson for international hacker group Anonymous appeared in a video to demand the scrapping of a law requiring local news websites to obtain annual licences.

Singapore strictly regulates the traditional media, but insists the licensing rules enacted in June 2013 do not impinge on Internet freedom.

None of the hackers convicted or facing trial have commented on any ties with Anonymous.

Singapore in August announced new measures to boost cyber-security following the attack on government websites.

It is upgrading its Cyber-Watch Centre, allowing it to track malicious activities and respond swiftly to security threats.