Nine people have died of bird flu in China this year, state media reported Wednesday, after the World Health Organization (WHO) urged all countries to promptly report human infections.

Disease control centres in three provinces including northern Henan, central Hunan, and southern Guangdong have reported at least nine deaths from the virus, the official Xinhua news agency reported.

The most recent cases came from Henan, where local health authorities reported deaths of two restaurant workers on Tuesday.

China has reported over 1000 cases, with 38.5 percent of them resulting in death, since an outbreak started in March 2013, according to figures from the WHO.

The bird flu strain H7N9, which can cause serious respiratory illness, strikes most commonly in winter and spring.

So far this season, infections have been reported across southern China, including in major cities Shanghai and Hong Kong, Xinhua said.

Bird flu scares in the past two years have seen mass culls of up to tens of thousands of birds in Hong Kong, including as recently as June.

H7N9 is a particular worry for authorities as it does not kill infected chickens or cause them to develop symptoms, which allows it to spread undetected until contact is made with humans.

The "epidemic" began last September, but there has been a "sudden and steep increase in cases" since December, WHO director general Margaret Chan said on Monday.

All countries are asked to "keep a close watch over outbreaks of avian influenza in birds and related human cases," Chan said, urging them to "detect and report human cases promptly".

The WHO has secured 350 million doses of vaccine in preparation for the next influenza pandemic, Chan said.

"The world is better prepared for the next influenza pandemic, but not at all well enough."