China's Health Ministry has confirmed another two people in Beijing are suffering from Severe Acute Respiratory Syndrome, bringing the total to four. One of the patients is in critical condition.
Health officials are calling it a "mini outbreak" of Severe Acute Respiratory Syndrome and are saying it likely started last month in a Beijing laboratory where researchers studied the virus.
Beijing health official Qiang Wei pledged to tighten laboratory procedures.
Mr. Qiang says that authorities are trying to prevent SARS from spreading.
China's Ministry of Health says it has identified another five suspected cases.
SARS, which can cause a serious pneumonia, started spreading in Southern China in late 2002 and went on to infect about 8,000 people worldwide. The cases were spread across more than a dozen countries, but most were reported in China and Hong Kong. At least 700 people died.
Dr. Julie Hall of the World Health Organization is leading an investigation into the latest outbreak in Beijing. She China is cooperating with the WHO.
"The [Chinese] ministry of health here has agreed to share information with WHO and samples will be going to the most appropriate laboratories over the next few days," she said.
The WHO suspects that the top Beijing lab likely had many lapses in safety, which led to at least two separate infections a month apart.
The U.N. health agency says those victims probably went on to infect more people. Hundreds of people who came into contact with the confirmed and suspected cases have been quarantined ahead of an important May 1 public holiday - a time when millions of people travel around China.
While both Taiwan and Singapore have reported isolated cases of SARS in laboratory workers last year, the Chinese cases are possibly the first reoccurrence of person-to-person transmission since the global outbreak ended last July.
Scientists earlier this year traced a handful of isolated SARS cases in China's southern Guangdong province to animal and rodent carriers.