THE BURNING of a Singapore national flag by Vietnamese protesters, who apparently mistook it for the flag of China, has drawn a sharp rebuke from the Singapore Ministry of Foreign Affairs (MFA) that called the offensive action "a serious incident".

Factories in two Singapore-run industrial parks, the Vietnam-Singapore Industrial Park (VSIP) I and II in Vietnam's south-eastern Binh Duong province, were on Tuesday torched and looted by anti-China protesters.

The violent mob was angry with China for deploying an oil drilling rig in disputed waters near the Paracels, a chain of islands claimed by both Vietnam and China in the South China Sea.

In a statement issued on Thursday, the MFA said it has verified that a Singapore flag was burnt by demonstrators in the VSIP I on Tuesday.

"This is a serious incident. The flag is a sacred national symbol and should be treated with respect," said the ministry.

It is believed that the protesters mistook the red-and-white Singapore flag that features five white stars and a white crescent for the Chinese national flag which is wholly red with five yellow stars.

The MFA said the Singapore Embassy in Hanoi has conveyed its strong concerns over the flag burning incident to the Vietnamese government and sought assurance from authorities that the incident will not occur again.

There have not been any reports of Singaporeans affected by the demonstrations so far, the MFA added.

The anti-China demonstrations have spread to 22 of Vietnam's 63 provinces, with at least one Chinese worker dead, the AFP reported.