China's premier on Sunday urged authorities to be vigilant against a new strain of bird flu that has killed 23 people, while saying that efforts to tackle the virus have so far been effective.

Speaking during a visit to the Chinese Center for Disease Control and Prevention, Premier Li Keqiang warned people to prepare for any new developments amid fears H7N9 could mutate into a form easily transmissible between humans.

"Countermeasures have been effective so far, but the situation is still developing as new cases turn up," Xinhua news agency quoted him as saying.

"We cannot afford to take it easy or relax, as we are facing a new virus," he said. "We should be prepared for any possible development."

He added that more effort was needed to diagnose and treat people with the virus as early as possible.

There have been well over 100 cases of the virus reported in China, with three more reported on Sunday, according to Xinhua, quoting local authorities.

The government announced on March 31 that the virus had been found in humans for the first time.

Most cases have been confined to eastern China and the only case to have been reported outside mainland China has been in Taiwan. The Taiwanese man was infected in China.

Experts have warned of the possibility of more cases over a wider area.

Chinese researchers, reporting in The Lancet on Thursday, said they had confirmed poultry as a source of the virus among humans.

Experts fear the prospect of the virus mutating into a form easily transmissible between humans, with the potential to trigger a pandemic.

The World Health Organization has said there has been no evidence of human-to-human transmission so far but warned that H7N9 was "one of the most lethal" influenza viruses ever seen.

Chinese health officials have acknowledged so-called "family clusters", where members of a single family have become infected, but have not established any confirmed instances of human-to-human transmission.