The World Health Organization has called on all countries to closely monitor outbreaks of deadly bird flu in birds and poultry, and to promptly report any human cases that could signal the start of a flu pandemic.

Different strains of bird flu have been spreading across Europe and Asia since late last year, leading to large-scale slaughter of poultry in certain countries and some human deaths in China. Experts fear the virus could mutate to spread more easily among people.

Nearly 40 countries have reported new outbreaks of highly pathogenic bird flu in poultry or wild birds since November, according to the WHO.

"The rapidly expanding geographical distribution of these outbreaks and the number of virus strains currently co-circulating have put WHO on high alert," WHO Director-General Margaret Chan told the UN agency's executive board on Monday.

The world is better prepared for the next influenza pandemic - following the H1N1 pandemic of 2009-10, which she described as "mild" - "but not at all well enough".

Chan said that under an agreement with drug makers, in return for countries sharing virus samples from which a pandemic vaccine would be derived, the WHO is promised 350 million doses of vaccine for distribution.

"We cannot allow so many countries to be without tools," Chan said later. "Remember, it takes four to six months to get the vaccine."

China has had a "sudden and steep increase" in human cases of H7N9 since December and the WHO has not been able to rule out limited human-to-human transmission in two clusters of cases, although no sustained spread has been detected thus far, she said.

Chinese health officials reported several cases of H7N9 in regions including Shanghai and Guangdong, Fujian and Anhui provinces. Two people died in Anhui in December.

China's delegation, led by Zhang Yang of the National Health and Family Planning Commission, told meeting participants that China would carry out its obligations on communicating and responding to any outbreaks.