TAIPEI -- Two more cargo pilots at Taiwan-based China Airlines were confirmed Monday as having COVID-19, bringing the total number of cases at the carrier to nine, the local epidemic-monitoring agency said.

The source of their infections so far remains unclear.

Both of the newly confirmed cases had flown to the United States on assignment earlier this month.

The island's epidemic monitoring authority has been expanding coronavirus testing to cover the infected pilots' contacts and the airlines' employees.

As of Monday, 514 pilots have received polymerase chain reaction tests, with 511 negative results and three positive results. Antibody tests have reported 507 negative results.

So far, Taiwan has recorded 1,104 COVID-19 cases, of which 12 have died.