It is still unclear how the Chinese soldier who died from bird flu earlier this month contracted the highly infectious H5N1 bird flu strain, Ministry of Health spokesman Mao Qun'an said here on Monday.

"Experts are analyzing the soldier's whole treatment history," Mao said at a press conference. "But they are still in the dark about how the soldier contracted bird flu."

"The investigation is ongoing," Mao added.

People who had been in close contact with the soldier have not shown any abnormal medical signs and are no longer under medical observation, according to Mao.

Last week, the Ministry of Health confirmed a Chinese soldier had died from bird flu on June 3, 10 days after he was diagnosed with the highly infectious disease on May 24.

China has reported a total of 25 human cases of bird flu since 2003, which have resulted in 16 deaths.