The World Health Organisation has called on all countries to closely monitor outbreaks of deadly avian influenza 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 slaughtering of poultry and some human deaths in China.

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 UN agency's 10-day executive board.

The new H5N6 strain causing severe outbreaks in Asia was created by gene-swapping among four different viruses, she said.

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

In China, there has been 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 human cases although no sustained spread has been detected thus far, she said.

Under the International Health Regulations, a binding legal instrument, 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."