BEIJING, Dec 10 (Reuters) - Chinese health authorities said on Monday they were hunting for the causal link between a son and his father both struck by bird flu but have found no evidence that the virus has mutated into a new strain.

The 52-year old father was diagnosed with the H5N1 strain of bird flu late last week in the eastern province of Jiangsu, days after his 24-year old son died from the disease.

This rare case of two family members struck by the disease has drawn urgent concern from health authorities, because humans almost always contract H5N1 from infected birds. Experts fear the virus could mutate into a strain that jumps easily from person to person, risking wider outbreaks.

Chinese Health Ministry spokesman Mao Qun'an said analysis of a sample taken from the dead son indicated the bird flu virus has not mutated, but he could not exclude the possibility of human-to-human infection in this case.

"The virus is still avian and has not undergone a mutation in its nature," he told a news conference.

Mao said one of the men might nonetheless have infected the other through close contact, or they might have become infected from another source or separate sources.

"We can't offer a final determination on these three possibilities," he said, according to a government Web cast (www.china.com.cn). "There is a thorough investigation under way."

Joanna Brent, spokeswoman for the World Health Organisation in Beijing, said it had no further information on the infections.

The official Xinhua news agency earlier reported that the son had had no contact with dead poultry and there had been no reported poultry outbreak in Jiangsu province.With the world's biggest poultry population and millions of backyard birds, China is at the centre of the fight against bird flu. There have been other cases of human infection without confirmed outbreaks among birds in the same area.

A Hong Kong newspaper controlled by the mainland said on Monday that the two men had both eaten chicken that was not fully cooked. The Ta Kung Pao paper cited unnamed sources as saying they had eaten the partly raw chicken in a restaurant in Nanjing, capital of Jiangsu.

The latest cases bring the number of confirmed human infections of bird flu in China to 27, with 17 deaths.

Spokesman Mao said the father was now in a stable condition and showing signs of improvement.