Story highlights The cases include 20 deaths, and 102 infection cases

Seventy patients are hospitalized with the virus

The new strain wasn't known until three weeks ago

International experts in China to monitor the H7N9 strain

The number of bird flu cases in China jumped Sunday to 102, including 20 deaths, the World Health Organization announced.

Seventy patients remain hospitalized with the virus. The WHO said there is still no evidence of human-to-human transmission.

So far the virus has mainly affected eastern China, with 11 deaths and 33 infection cases reported in Shanghai, 24 cases, including three deaths, in Jiangsu Province, 38 cases, including five deaths, in Zhejiang Province, while Anhui Province has confirmed three cases, with one ending in death.

Further afield, four people -- including one in Beijing in northern China and three in central Henan Province -- have tested positive for the H7N9 virus.

The current strain of bird flu, H7N9, was not detected in humans until last month.

A team of international experts are currently in China conducting a week-long assessment of the virus, the WHO said on Friday.

Map: Bird flu spread

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"Right now it is still an animal virus that rarely infects humans," Dr. Michael O'Leary, the head of the WHO's office in Beijing, told reporters.

On Thursday, the central government suspended wild bird sales to try to prevent the spread of the virus, although many questions remain as to the source of infection. It follows a ban on live poultry trading in affected provinces. A large number of birds have also been slaughtered, state-run Xinhua said.

O'Leary said there was legitimate reason for concern about the new virus, but suggested it was premature to begin mass culling of poultry.

"I eat chicken every day," O'Leary said with a laugh. "Chicken is of no concern at all."

Until March, the virus had only been present in birds, which is why they've become the focus of the investigation.

However, 40% of patients with H7N9 had not come into contact with poultry, according to the Chinese Center for Disease Control and Prevention.

Authorities are continuing to monitor more than 1,000 people who have come into close contact with confirmed cases.