He stressed that border restrictions would be imposed only if there is evidence the H5N1 strain moves into the human population.

"If it is proven to be human-to- human transmission, then we have to be very careful and we might have to close the border," Chow said in Beijing.

"But that won't happen immediately. We need to have some time to investigate to be sure that this is a new virus and has the risk of human-to- human transmission."

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Chow's statement came as Xinhua News Agency quoted Vice Premier Hui Liangyu as saying: "Our country is at the peak season of the bird flu outbreak. The situation is grave."

Hui issued a directive for officials to go all out to prevent the spread of the virus, Xinhua said.

"We cannot let down our guard, we cannot underestimate the risks of the outbreaks," Hui warned.

Quarantine measures must be taken and surveillance must be stepped up to prevent the disease from spreading to humans, Hui was quoted as saying.

Many countries have said they would close their borders in the event of a human-to-human outbreak, but for Hong Kong the commitment is remarkable since its border is one of the busiest in the world.

According to the Census and Statistics Department, a staggering 91 million people entered Hong Kong through air, sea and land checkpoints last year - an increase of almost 30 million from 1999.

Of that, 12.4 million arrived by air, 9.9 million by sea and 68 million people channeled through the borders, with 60 million of those people from the mainland.

To prevent the spread of the bird flu virus, the SAR and mainland governments will strengthen cooperation to combat the smuggling of food, according to an agreement signed Thursday by Chow and the director of the General Administration of Quality Supervision, Inspection and Quarantine.

The move follows a series of crises caused by tainted mainland food imports such as fish and eels, which were found to contain traces of the banned anti-fungal agent malachite green, and pigs, which have been hit by a streptococcus suis infection.

Chow said the agreement included better communication and notification between the SAR and the mainland, and exercising greater control over imports.

The agreement came a day after China announced its first reported outbreak of bird flu in more than two months, saying 2,600 birds had died on a farm in Hohhot, the capital of Inner Mongolia.

Chow said people should not panic since most chickens consumed in Hong Kong are imported from Guangdong and are inspected daily at the border.

"All chickens imported to Hong Kong from Guangdong are vaccinated. Our staff continually check the blood serum of 200 imported live chickens at Man Kam To each day to see if they have the antibody to the bird flu virus. We have not found anything wrong over the past two years," he said.

Chow said the SAR government will regularly review the effectiveness of the measures to monitor bird flu.

"We will keep a close watch on the situation and we will not minimize the risks. We will take a close look at the mutation of the virus to decide whether we need to change our strategies," he said.

The director of the General Administration of Quality Supervision, Inspection and Quarantine, Li Changjiang, said there had been cases in the past where infected chickens had been smuggled into the mainland from outside the country.

He said measures will be taken to combat this smuggling of live chickens to or from the mainland to other places, including Hong Kong.

Li said Hong Kong will also be notified by the mainland about any outbreak of an animal disease.