Two suspected Sars patients in Beijing were confirmed as having the disease on Thursday and one was in critical condition, China's Health Ministry said.
Several countries have put up their guard against the spread of Sars and China said it had notified foreign visitors that a Beijing disease control laboratory could be the source of the new outbreak.
"Two suspected Sars patients in Beijing were confirmed as having Sars and one is critical," the Health Ministry said, adding that all confirmed cases were traceable to the same Beijing laboratory
Hundreds of people have been isolated for observation.
A laboratory leak led to the first death this month from Sars since the previous outbreak. A total of four diagnosed and five suspected cases have been found in eastern Anhui province and in Beijing.
But as hundreds of millions of Chinese prepared to set off for the weeklong May Day holiday, the World Health Organisation (WHO) said the threat was not great enough to require travellers to cancel their plans.
In Beijing's main railway station and airport, virtually no one was wearing face masks, ubiquitous last year during the outbreak of the deadly disease that killed nearly 800 people world-wide.
Vietnam, where five people died last year, has resumed checks on temperatures of passengers from China, a newspaper said, following a similar move by Thailand. Canada has introduced voluntary screening.
China's Health Ministry said 18 people from countries including Australia, Russia, South Korea and Japan had visited the Institute of Virology of the Chinese Centre for Disease Prevention and Control in Beijing.
"The institute is where one of the diagnosed patients and one of the suspected Sars patients worked," the China Daily said.
"The ministry has informed visitors, countries concerned and the WHO of relevant information and no abnormal problems have been reported."
Dr Julie Hall, the WHO's Sars team leader, saw no need for people to stay at home.
"We don't believe that there is any reason to cancel or delay any travel plans that they might have had for the upcoming holidays," she told reporters. "But clearly we will be looking at this on a daily basis."
She said she believed "well over" 1,000 people had been identified as at risk and isolated in the latest outbreak.
China has won praise for its Sars surveillance, reporting and co-operation since a cover-up of last year's outbreak.