China is confident it can control an outbreak of a new strain of bird flu, a senior Chinese health official said on Sunday as the World Health Organization (WHO) said there had now been 21 human cases of the H7N9 flu with six deaths.

China has said it is mobilizing resources nationwide to combat the new strain, monitoring hundreds of close contacts of confirmed cases and culling tens of thousands of birds where traces of the virus were found.

“We are confident we can effectively control it (H7N9),” the head of China’s National Health and Family Planning Commission Li Bin told Reuters on the sidelines of a World Health Organization-backed event in Beijing.

Li did not elaborate, but she is the most senior Chinese health official yet to publicly comment on the subject.

In an update from its headquarters in Geneva, the WHO said more than 530 close contacts of the 21 people now listed as confirmed cases are being closely monitored for any signs of similar disease.

“At this time there is no evidence of ongoing human-to-human transmission,” it said.

The bird flu outbreak has caused global concern and some Chinese internet users and newspapers have questioned why it took so long for the government to announce the new cases, especially as two of the victims fell ill in February.

The government has said it needed time to correctly identify the virus.

The WHO’s representative to China, Dr. Michael O’Leary, repeated that no evidence of transmission between humans has been found and praised China for its efforts to determine the source of the virus.

“I’m very impressed with the action of the laboratories in this regard,” O’Leary said at a World Health Day event in the Chinese capital.

“China is demonstrating their ability to get on top of this problem quickly,” he said.

In 2003, authorities initially tried to cover up an epidemic of Severe Acute Respiratory Syndrome (SARS), which emerged in China and killed about 10 percent of the 8,000 people it infected worldwide.

Other strains of bird flu, such as H5N1, have been circulating for many years and can be transmitted from bird to bird, and bird to human, but not generally from human to human.