WHO also warned that SARS has become more serious in the Taiwanese capital of Taipei because officials cannot track down the source of infection for a high number of people - the latest sign the island was losing control of the outbreak.
China reported five new SARS fatalities and 85 new cases Saturday, raising the mainland's death toll to 235. Worldwide, SARS has killed at least 526 people and infected 7,200 others in more than 25 countries.
"Right now, the situation is that we have a whole load of people (in Beijing) and we don't know where they got the disease," WHO spokeswoman Mangai Balasegaram said. "The problem with the data is that there are holes in it."
It is not exactly clear why information is so lacking about Beijing's SARS patients, Balasegaram said. But she suggested that some hospitals and health workers had not been trained to question patients correctly.
"That means you don't understand what's going on," she said. "The epidemic might be flying off in one direction, and you might not know about it."
Balasegaram said health officials need to find out "where the people were living, where they went and who they contacted," adding that Beijing was improving its data.
China has been criticized for dragging its feet on releasing information on SARS after the disease struck the country in November.
During the early stages of the epidemic, Taiwan was able to quickly trace and isolate the infection sources, many of whom were people who recently visited China and Hong Kong. But recently, SARS has spread rapidly within communities.
Taiwan reported 23 new cases Saturday, its largest one-day jump since the outbreak began there two months ago. Authorities sealed off a housing complex where one resident died of the virus, while one of the largest department stores in Taipei was closed for a major disinfection following the infection of a cashier.
The State Department said Americans living in Taiwan may wish to consider leaving.
On its Web site, WHO lumped Taiwan together with Chinese cities and regions reporting a rapid transmission of severe acute respiratory syndrome. They included Hong Kong, Beijing, the southern province of Guangdong and the northern province of Shanxi.
WHO said the SARS danger in China could be shifting to the poorer countryside, where a shortage of hospitals and doctors could make an epidemic disastrous. A four-member WHO team was investigating an outbreak in the northern province of Hebei, which borders Beijing.
In the Chinese capital, city officials lifted quarantines on six hospitals, two residential communities and four construction sites where SARS cases were discovered. Authorities also said 80,000 students would be allowed to gather in Beijing next month for college entrance exams.
In a further show of optimism, China's official media reported Saturday that the outbreak will not delay the country's first manned space launch. A front page story in the Communist Party newspaper People's Daily did not give a date for the launch of the Shenzhou No. 5, but earlier reports put it in the second half of this year.
In other developments Saturday: