China is strengthening prevention and control of bird flu although no new human cases have been confirmed for more than a month, a health official said Monday.

"We are still keeping a close eye on bird flu and have strengthened scientific research and nationwide surveillance," said Ministry of Health spokesman Mao Qun'an.

So far, the virus outside of the human infections had show no sign of mutation, and human-to-human transmission was still unproved in clinical studies, he said.

China had enhanced research in human vaccines and drugs as well, he added.

Monitoring of unexplained pneumonia cases had also been strengthened by the ministry, which required local health institutes to screen pneumonia cases and report immediately to health authorities cases with unknown causes, Mao said.