The rules will ensure "faster and better detection and evaluation of health emergencies with an international scope, those that have the potential to cross frontiers," said Guenael Rodier, the World Health Organisation's IHR coordination director.

The WHO said the emergence of SARS (Severe Acute Respiratory Syndrome), the deadly Ebola Virus in Africa and human cases of bird flu in recent years demonstrated the speed with which lethal diseases could appear and spread.

"SARS was a wake-up call for all of us. It spread faster than we had predicted and was only contained through intensive cooperation between countries which prevented this new disease from gaining a foothold," said WHO Director General Margaret Chan.

"Today, the greatest threat to international public health security would be an influenza pandemic."

"The threat of a pandemic has not receded, but implementation of the regulations will help the world to be better prepared for the possibility of a pandemic," she added.

The rules bolstering international cooperation to stifle health emergencies were agreed by the World Health Organisation's 192 member states in 2005.

Countries will have an obligation to alert the WHO about health threats that might have international implications, ranging from disease to contaminated food, chemical agents or radioactive material, the WHO said.

It also lays out action like quarantines, surveillance of travellers and a requirement for a 24 hour, seven day a week alert system.

The revision was partly motivated by fears that easier global air travel can help spread infections far and wide in short space of time.

SARS reached seven countries on three continents within the space of weeks in 2002 and 2003, and eventually blossomed to 8,096 cases in 29 countries before it petered out, according to the WHO.

The previous regulations last updated 1969 in were limited in scope to four named diseases -- cholera, plague, yellow fever and smallpox -- while the revised ones are open-ended and place individual countries under greater pressure to provide transparent information.

"The world is such now that these events cannot be obscured," said Michael Ryan, the WHO's director of epidemic alert and response.

In recent years, several countries including China were criticised for being slow or lacking transparency in the initial phases of SARS and bird flu, amid claims they failed to reveal the onset of then largely unknown illnesses to their neighbours.