GENEVA - The World Health Organization called on all countries on Monday to monitor closely outbreaks of deadly avian influenza in birds and poultry and to report promptly 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 slaughtering 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 avian influenza 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," Margaret Chan told the start of the U.N. agency's executive board.

The world is better prepared for the next influenza pandemic - following the H1N1 "mild" pandemic in 2009-2010 - "but not at all well enough", she said.

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

"We cannot allow so many countries to be without tools," Chan later told Reuters. "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 spread in two clusters of cases although no sustained spread has been detected thus far, she said.

Under the International Health Regulations, WHO's 194 member states are required to detect and report human cases promptly, Chan said, adding: "We cannot afford to miss the early signals."

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

"Currently H7N9 overall statistics remain the same," Zhang said. "China will continue to strengthen its cooperation and exchange with WHO in this regard."

David Nabarro, an international public health expert and one of six candidates to succeed Chan in the top WHO post, said that addressing the threat of avian flu jumping the barrier to pose a serious threat to humans was a "central priority".

"This group of viruses are persistent in moving between wild birds and poultry. We should always have a good high guard and never be complacent," Nabarro, a former U.N. coordinator for avian and human influenza, told Reuters.