The warning to avoid travel to the regions hardest hit by severe acute respiratory syndrome, or SARS, was the first such advisory the WHO has ever issued because of a disease outbreak. Earlier travel warnings were in response to wars or other conflicts.
"We have decided to make a recommendation that people planning to travel to Hong Kong and Guangdong consider postponing," said David Heymann, WHO's head of communicable diseases in Geneva.
The travel advisory the U.S. Centers for Disease Control and Prevention issued on March 29 is more broad than the WHO's recommendations and advises postponing trips to all of mainland China as well as Hanoi, Vietnam and Singapore.
The WHO also announced Wednesday that China has agreed to allow the U.N. agency's experts to visit Guangdong, where the disease most likely originated.
"These are very positive steps taken today by China," Dr. Heymann said Wednesday. "As a result we?ll be able to gather even more evidence about the nature of the SARS outbreak in China."
The permission came as international criticism of China's reaction to SARS grew -- and as China released more information on the disease's path in the south last month.
In a statement faxed to news organizations Wednesday, the Guangdong provincial government said it had 361 new cases in March. The cases in China are concentrated in Guangdong, but there are now reports that the disease has spread to other parts of the country.
U.S. Secretary of Health and Human Services Tommy Thompson said Tuesday more pressure would be applied on China, where the disease was apparently kept quiet for months.
"It's important to see if we can work in more of a collaborative fashion with China," said Thompson. "I think we're going to put on more pressure."
The State Department has responded to the continued spread of SARS by offering free flights home to non-emergency U.S. diplomats in Hong Kong and Guangzhou. Susan N. Stevenson, spokeswoman at the Hong Kong consulate, said no American there has become ill with the disease.
Also in Hong Kong, some 240 residents of an apartment complex where SARS has spread were taken to quarantine camps Tuesday.
Secretary Thompson said such measures don't yet appear warranted in the United States.
"If there is a virus that is explosive ... and the only way to control it is by quarantine, we have to consider it," he said Tuesday. "But we're not there yet."
So far, four percent of SARS victims have died. While the rest should recover, many develop severe pneumonia that requires hospitalization for a few weeks and treatment with strong medication, which can cause birth defects.
Over 2,200 people worldwide have been infected by the virus, which originated in Guangdong in November.