SEOUL (Reuters) - South Korean health authorities aim to test more than 200,000 members of a church at the center of a surge of new coronavirus cases as President Moon Jae-in said on Tuesday the situation was “very grave”.

South Korea’s tally of cases of coronavirus cases rose to 977, fuelling fears the outbreak, which is thought to have begun in the central Chinese city of Wuhan in December, is developing into a pandemic.

A member of a Korean Air cabin crew tested positive for the virus, the airline said, prompting it to shut its office near Seoul’s Incheon International Airport, where the crew briefing room is located.

About 68% of South Korea’s cases have been linked to the Shincheonji Church of Jesus, where the first case was reported in a 61-year-old woman, but it is not known how she became infected.

South Korea reported its eleventh death from the virus as a 36-year-old Mongolian national died on Tuesday, becoming the first foreigner there to have died from the virus. The Korea Centers for Disease Control and Prevention (KCDC) reported 144 new confirmed cases.

Many of the new cases were in the southeastern city of Daegu, where the church is located, and from nearby North Gyeongsang Province, the KCDC said.

Moon, in his first visit to Daegu since the outbreak began, called the situation very grave and said the coming week would be key in the battle to contain the virus.

He reassured residents that the government was not considering locking down the city.

Authorities said they had tested 13,000 people on Monday and expected to test another 12,500 on Tuesday, up from about 7,500 a day previously.

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Each polymerase chain reaction (PCR) machine that South Korea is using to test for the virus is running four tests a day, up from three in recent days, the KCDC said. Each test takes two to three hours.

The leader of the Shincheonji Church said it had agreed to give authorities the names of all members in South Korea, estimated by media at about 215,000 people.

The government would test all members as soon as possible to “to contain the spread of the virus and relieve public anxiety”, the prime minister’s office said in a release.

U.S. WARNING

Health officials said the first priority was to test about 1,300 of more than 9,200 members of the Daegu church who are showing symptoms. Those tests should be done by Wednesday.

Church members in Daegu had been put under mandatory quarantine, KCDC Director Jeong Eun-kyeong said.

The church, which has faced criticism of its handling of the outbreak, asked the government to ensure the personal details of its members did not become public.

The government’s goal is to stabilize the situation in Daegu within four weeks, Vice Health Minister Kim Kang-lip said.

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North Gyeongsang Province has also seen surges in cases, most from a hospital in Cheongdo, and it was designated a “special care zone” along with Daegu last week.

On Tuesday, 21 of the 33 cases from the province were confirmed to have come from a home for the disabled people in Chilgok County, also near Daegu, traced to a worker there whose mother is a member of the Shincheonji Church in Daegu, Governor Lee Cheol-woo said.

The U.S. Centers for Disease Control and Prevention, meanwhile, raised its warning level for South Korea and recommended Americans avoid all non-essential travel to the country, citing the “widespread, ongoing outbreak” of the coronavirus.

The U.S. and South Korean militaries said they were considering scaling back joint training, in one of the first concrete signs of the virus’ fallout on global U.S. military activities.

South Korean Defense Minister Jeong Kyeong-doo told a news conference in Washington 13 cases had been confirmed in the South Korean armed forces.