The agency issued the warning despite praising China's handling of outbreaks in its vast poultry flocks and the country's first human cases reported Wednesday.
"We expect there will be more poultry outbreaks," said Henk Bekedam, the chief WHO representative in Beijing. "In this cold weather, the virus can survive longer in the climate and therefore have a bigger chance to infect poultry.
"The moment you have more poultry outbreaks, you will also expect more humans to be exposed to the poultry and therefore it (human cases) can indeed happen," he said.
Two more human deaths from bird flu were reported in Indonesia on Thursday.
Tests sent to a Hong Kong laboratory for a 20-year-old woman and a 16-year-old girl came back positive for the H5N1 virus, said Sari Setiogi, a WHO spokeswoman.
They died last week in the capital Jakarta after coming into contact with sick chickens, she said.
Bird flu has ravaged poultry stocks across Southeast Asia since 2003, jumping to humans and killing at least 64 — a toll that does not include Indonesia's latest deaths. Most of the deaths have been in Vietnam and Thailand, but Indonesia's caseload is steadily rising.
Most human victims have contracted the disease from sick birds, but experts fear the virus will mutate into a form that is easily transmitted between people, sparking a possible pandemic that could kill millions.
China's Health Ministry announced Wednesday that it had confirmed two human cases of the virulent H5N1 strain of bird flu — a 24-year-old woman who died and a 9-year-old boy who recovered.
It said the boy's 12-year-old sister, who died, was a suspected case.
Asked whether there was any sign that China was at risk for a pandemic, Bekedam said, "If there's evidence of human-to-human transmission, there will be small clusters. We are not at that stage."
While China is "extremely committed" in its fight against bird flu, he said it was necessary to be vigilant.
In response to the Chinese cases, Hong Kong on Friday will begin checking the temperatures of people crossing the busy border with mainland China, officials said.
Officials will use 11 infrared machines that will screen the temperatures of travelers at the two border points, said Cindy Lai, assistant director at the Health Department.
Travellers with a fever will be assessed for flu symptoms and possibly referred to hospitals for a further check-up, the statement said.
In Vietnam, Prime Minister Phan Van Khai approved another $82.6 million for the fight against bird flu, and the government announced that its first large-scale emergency drill will take place later this month.
The first of three major exercises that will test pandemic preparedness will be held in Hanoi on Nov. 27, the official Vietnam News Agency said. Some 1,100 people, including health officials, police and military forces, will take part in the daylong drill, VNA said.
Vietnam has been battling the H5N1 virus since it emerged across poultry farms in late 2003 and has suffered two-third of the human deaths.
The country has taken increasingly tough measures against bird flu as the winter months approach, when the virus is most likely to spread.