Canadian officials expressed new concern today about the outbreak of a sometimes fatal respiratory disease as public health authorities revealed that dozens of members of a Roman Catholic church group may have become infected at a prayer service.

The health department asked more than 500 members of the church group to enter quarantine out of concern that they could spread the disease, known as severe acute respiratory syndrome, or SARS. Officials said there are 10 probable cases of SARS among the group and 19 suspected cases. They said two physicians who treated them became ill.

"We are concerned about the extent of the illness in this community," said Sheela Basrur, Toronto's chief medical officer. Basrur said one member of the group could trace the disease back to Scarborough Grace Hospital, the so-called epicenter of the outbreak in Toronto. She said some church members attended the wake of a SARS victim who had been treated at Scarborough Grace. Several relatives of the victim attended the wake and were suffering from symptoms, Basrur said.

Canada, which has the highest number of SARS cases outside Asia, is desperately trying to contain the spread of the disease, which has killed 13 people in this country since March 5, when a woman with SARS returned home from a trip to Hong Kong. As of Monday, health officials reported 287 probable and suspected cases in Canada. That number includes 100 probable and 136 suspected cases in the province of Ontario.

For weeks, officials have cautioned the public against panic, saying that all known SARS cases could be tracked to two clusters: people who have traveled to Asia or their relatives, and health care workers who treated them. On Monday, officials said they may have a third cluster: people who belong to the Catholic group. Officials said they were trying to track down people who may have come into contact with the members. They also expressed concern that church members were breaking quarantine.

The church group has been identified as BLD Covenant Community, a charismatic Catholic organization based in Manila. BLD stands for Bukas-Loob Sa Diyos, which means "open in spirit to God."

The group's Toronto leader says that since the group received information about infections, it has suspended all special gatherings and events in Toronto. It has asked those in quarantine not to participate in the celebration of Easter, the most important celebration in the Catholic Church.

In a message to its members, it reminded them that they must adhere to quarantine rules. "We have promised to be faithful to God's direction and to listen to those placed in authority such as Toronto Public Health," the message said. "It is part of our commitment to do whatever we are required to do to help stop the spread of the disease that has already damaged so many lives and hurt so many families."

As the disease spreads, health officials said, it is more likely that infections will occur without being traced to a single source. Officials here and in other countries have been trying to isolate and quarantine all cases they can identify.

Today, officials in Toronto again urged those who have been asked to go into isolation to stay home. Colin D'Cunha, Ontario's commissioner of public health, said: "With the holiday weekend approaching, there are family and religious gatherings that people in home isolation may want to attend. However, I strongly urge anyone still in quarantine to refrain from going to these gatherings, if you have not been home for a full 10 days or if you have become sick."