"Of the new suspected cases each day, about 70-80 per cent are from the people under monitoring in hospital fever clinics for medical observation and isolated people who have had close contact with confirmed SARS cases," said Liang Wannian, vice-director of the Beijing Municipal Health Bureau, in an exclusive interview.

The remaining 20-30 per cent of suspected SARS patients are people from outside Beijing who have come to the capital for medical treatment and other unidentified sources, according to Liang.

A larger proportion of confirmed SARS cases are from suspected cases.

Beijing reported 13 SARS cases yesterday, among which 10 were previously suspected cases.

This is in contrast to the situation on May 9 when just 40-50 per cent of the new SARS cases were patients already isolated by health authorities, according to the vice-director.

Figures released by the bureau said more than 20 per cent of suspected cases were later confirmed as SARS cases.

In another development, four people suspected of contracting SARS who work at two Beijing-based factories run by Japanese electronics giant Matsushita have been given the all-clear.

"They have most probably contracted measles," Liang said.

No new SARS case has been found since May 17 when five confirmed cases were reported there.

There are still more than 40 workers suffering from fevers.

"This case tells us that we should attach great importance to distinguishing SARS from other respiratory diseases in our diagnosis," Liang said.

The two factories that produce colour TV kinescopes and lights under the Panasonic brand have been closed temporarily after the epidemic among workers, according to sources with Matsushita Electric (China).

The Matsushita factories at Dashanzi in Chaoyang District are the only two foreign-invested enterprises that have closed due to SARS fears in Beijing.

It is expected that the two factories will reopen this week if no new cases emerge, according to Wang Ge, an official with the firm.

The five confirmed SARS patients are all Chinese migrant workers. None of the local workers at the factory, who live with their migrant counterparts in the same dormitory, have fallen ill, according to Liang.

Beijing municipal government officials have taken immediate measures to disinfect the factories.

A total of 53 people who were in close contact with the SARS patients have been quarantined.

Medical staff are trying to identify the source of the SARS cases and bring the situation under control.

Matsushita Electric has established an overseas crisis management commission to deal with the disease with a company vice-president as the chief.

Beijing has seen a "notable downward" trend of SARS cases over the past week owing to the government's effective disease control and treatment measures, a municipal official said on Saturday.

Cai Fuchao, the city's publicity head, told a press conference on Saturday that the number of SARS patients in the city dropped markedly between May 17-23, with the average number of confirmed SARS cases decreasing to 13.2 per day.

The daily average number of health workers infected with the SARS virus dropped from 15.81 between April 21-May 1 to 1.1 last week.

The municipal government is now giving priority to preventing the spread of the disease among migrant workers, Cai said.

Over the past week, probable and suspected SARS cases among migrant workers in the city reached 15 and 33 respectively, topping the list of all SARS patients reported in the week in terms of their occupation.