His comments came as China announced its lowest nationwide daily increase in infections in weeks.
The Health Ministry reported four new SARS deaths and 39 new cases — a fraction of the increase announced in early May when China was reporting more than 150 new cases a day. Severe acute respiratory syndrome has killed 275 people on the Chinese mainland and infected 5,191.
Beijing, the hardest hit area in the world, reported one new death and 28 new cases. That raised the city's total reported cases to more than 2,400.
However, WHO expert Dr. Daniel Chin said city hospitals aren't recording some patients who have SARS symptoms but had no known contact with an infected person and recovered quickly.
"In the last couple of days, we have become concerned that there actually is underdiagnosis of probable SARS cases in Beijing," Chin said. However, he said it didn't appear that hospitals were trying to hide cases.
Chinese leaders have promised to report honestly on SARS and have threatened to punish officials who try to conceal cases.
State media quoted Premier Wen Jiabao calling for stern enforcement of anti-SARS restrictions and said more than 300 local officials have been punished for negligence in disease fighting work.
In Jiangsu, in China's east, a woman was sentenced to one year in a labor camp for leading villagers who attacked an office building that was being converted into a SARS quarantine center, the state news agency said.
On Thursday, state media publicized a warning by China's Supreme Court that people who cause death or severe illness by knowingly spreading SARS could face prison terms or possible execution. The court said quarantine violators could be sentenced to up to seven years in prison.
In Taiwan, the health chief resigned to take responsibility for the worsening SARS crisis as 10 new cases of severe acute respiratory syndrome were reported, along with three deaths that included the first two doctors in Taiwan to be killed by SARS. The disease has now sickened 274 people and killed 37 in Taiwan.
One of the Taiwan's top SARS fighters warned that another wave of cases was about to erupt. "We can expect another wave of SARS cases infected after May 10," Lee Ming-liang, head of the government's SARS Control Committee, said Friday.
In Hong Kong, World Health Organization investigators said that leaky sewage pipes and bathroom ventilation fans carried contaminated droplets through parts of an apartment complex, causing one of the world's worst outbreaks of SARS.
More than 300 people came down with the illness at the Amoy Gardens apartment complex in late March, and 35 people died. The speed of the infection amazed health experts, who thought the disease spread mainly by person-to-person contact.
A report written by a team of WHO investigators blamed an "unlucky" combination of circumstances — a patient with diarrhea, seeping pipes and drafty air shafts.
Apparently, the disease spread when a man with SARS visited his brother at the complex. The man had diarrhea. When he used a bathroom with a broken toilet, droplets containing the virus apparently got into exhaust fans that spewed them into nearby apartments.
In other developments:
The travel industry around the world will lose 6.9 million jobs this year because of SARS fears coupled with economic gloom, predicted the World Travel and Tourism Council.