Bird flu tests on three dead baby chickens found in the luggage hold of a flight from Fujian province on Wednesday came back negative, the government said on Thursday, amid a spike in human infections in mainland China.

According to the Airport Authority and the Agriculture, Fisheries and Conservation Department (AFCD), the three chicks were discovered after Shenzhen Airlines ZH9091 landed in Hong Kong from Quanzhou at 10:30 a.m. on Wednesday.

The Airport Authority informed the Port Health Office under the Department of Health soon afterwards and the plane was then sterilised.

The birds were sent to the AFCD's Tai Lung Veterinary Laboratory in Sheung Shui for bird flu tests, which found no trace of the disease.

The AFCD said that, under the Public Health (Animals and Birds) Ordinance, anyone who brings live birds or poultry into the city must get health certificates for them.

Anyone who fails to do so could be fined HK$25,000 and have the animals confiscated.

The Centre for Health Protection of Hong Kong announced on Wednesday that it would conduct bird flu tests at the city's wet markets "very soon" to try to strengthen protections against the virus, which has infected many on the mainland.

The number of human cases on the mainland this winter had reached 364 by Tuesday, surpassing the usual 200 or so cases reported each winter in the past few years.

Four human cases, including two fatalities, have been reported in the city so far, and all of the infected had recently travelled from the mainland.Health officials said they would also step up patrols at the 29 licensed poultry farms in the city. If any samples tested positive, officials said they would halt sales or suspend farm trading.

The Centre for Health Protection said the latest samples from live poultry markets on the mainland showed the contamination rate appeared to be rising.

Around 10 percent of samples taken from Guangdong farms in January tested positive for the H7 virus behind bird flu, while 15.8 percent tested positive in Jiangsu province, the centre said.