Geneva, Jumada I 26, 1438, February 23, 2017, SPA -- The World Health Organization says an increase in bird flu cases in China this year has not shown sustained human-to-human transmission, but it vows to remain "vigilant" over the puzzling outbreak in which affected fowl don't show any symptoms. Spokesman Christian Lindmeier of the U.N. health agency says the outbreak of the H7N9 strain is the fifth since 2013 in China, causing at least 73 deaths and 425 confirmed cases. Chinese authorities have put the death toll even higher. Lindmeier said Tuesday that health officials have reported three clusters in China "where limited human-to-human transmission could not be completely ruled out." Chinese authorities have ordered the closure of live poultry markets in its south-central regions. Most reported cases have emerged in densely populated areas between Shanghai and Hong Kong. -- SPA 01:54 LOCAL TIME 22:54 GMT www.spa.gov.sa/w338601