No mutation of H7N9 bird flu has been monitored yet and transmission from animals remains the only way in which humans have picked up the virus, China's health authority said on Tuesday.

Speaking at a press conference, Yao Hongwen, spokesman for the National Health and Family Planning Commission, noted that there have continued to be sporadic reports of H7N9 infections.

China confirmed three new cases this month, with two in Hong Kong and one in east China's Zhejiang Province, and Yao said that local authorities have tightened prevention measures.

The spokesman promised that the commission will continue to strictly control infection sources and release relevant information on a timely basis.

The mainland has so far reported a total of 140 H7N9 cases, including 87 patients who have recovered.