NITED NATIONS, April 15 EMonkeys experimentally infected with a new coronavirus have developed an illness similar to the mysterious human respiratory disease SARS, and it is now almost certain that the coronavirus causes the disease, a World Health Organization official said here today.

Dr. David L. Heymann, executive director in charge of communicable diseases for W.H.O., said the agency "is 99 percent sure" that SARS is caused by the new coronavirus based on the monkey experiments in the Netherlands. Experiments on animals are necessary because the lack of an effective treatment for SARS and the relatively high death rate make it unethical to conduct such experiments on humans.

Preliminary findings show that the monkeys developed an illness resembling SARS after the coronavirus was put in their nostrils. Some monkeys developed pneumonia, and examination of their lungs under a microscope showed that the coronavirus caused a pattern of lung damage similar to what affected humans have suffered.

Scientists from the W.H.O.'s network of 12 international laboratories who have been seeking the cause of SARS will meet Wednesday in Geneva and by teleconference to review the evidence concerning the new coronavirus.

The monkey experiments are essential in fulfilling the steps known as Koch's postulates that are needed to establish proof that a virus or other microbe causes a disease. Applying the postulates to SARS, scientists must determine whether injecting the coronavirus into animals causes similar symptoms to those that humans experience. A formal announcement that the likely cause of SARS has been found could come as early as Wednesday.

Verifying the cause of SARS is essential for the development of reliable diagnostic tests to determine who has the disease so that affected patients can be treated in isolation and those who are not affected can carry on with their normal activities. A principal aim of the W.H.O meeting on Wednesday is to discuss how close researchers have come to developing such tests and to reach a consensus on their use in controlling the epidemic.

As of today, the disease has affected 3,042 people and caused the deaths of 154 of them in 22 countries and Hong Kong. The fatality rate has risen to 5.1 percent from 4 percent in recent days.

Dr. Heymann, in addressing United Nations delegates and staff today, expressed hope that new tests aimed at the coronavirus would eventually help contain SARS.

There is no determination yet whether SARS has the potential to cause epidemics around the world and become a permanent cause of disease like tuberculosis, Dr. Heymann said. "We can't make any predictions until we understand what is going on in China."

China has come under severe worldwide criticism for not fully reporting until recent weeks the number of SARS cases that have occurred there since November, and for not allowing international teams of experts to visit affected areas until recently.

Despite China's pledge to report SARS cases fully, in recent days Chinese doctors have contended that they have treated many cases in military hospitals that the Chinese government has not reported to W.H.O. A team of experts from W.H.O. had been denied access to military hospitals. W.H.O. reported today, however, that its team of experts had visited one military hospital in Beijing and expected to visit others soon.

The Chinese government's decision to allow the W.H.O. team to visit military hospitals "is a welcome indication of China's willingness to come to terms with the SARS outbreak on the mainland," the organization stated. On Monday, China's president, Hu Jintao, said on state television that he was "very worried" about SARS.

Dr. Heymann said that China had developed a national SARS reporting system over the last three weeks and had elevated SARS to the status of two diseases, cholera and yellow fever, for which the government can impose quarantines.

Nine patients in Hong Kong died of SARS on Tuesday, setting a new single-day record for such fatalities. Five of those who died were younger than 45 and included a pregnant woman and four patients with no underlying illnesses.

On Tuesday, Shanghai imposed the first travel restrictions within China by ordering a halt to all group tours to Hong Kong.

As of today, doctors in the United States have reported 193 cases and no deaths. All but 19 of the cases have involved travelers to affected areas. The 19 cases represent secondary transmission to 14 family members and 5 health care workers.

Later this week, the Centers for Disease Control and Prevention in Atlanta is expected to lower the total of SARS cases in the United States to about 30. The C.D.C. has deliberately used a broader case definition than W.H.O. because federal officials wanted to cast a wide net to make sure they did not miss a mild case.

Over the weekend, W.H.O. added the United States to its list of SARS-affected areas. The agency took the step after the C.D.C. was criticized for not reporting secondary transmission of cases to W.H.O., as other countries have done.

The list includes Toronto; Singapore; China (Beijing, Guangdong, Hong Kong and Shanxi); Taiwan; Hanoi, Vietnam; and London. W.H.O. said that the United States and Britain had limited local transmission and there was no no evidence of international spread from those areas since March 15 and no transmission other than close person-to-person contact reported.