TOKYO — As the Diamond Princess cruise ship steamed back into port in Yokohama, Japan, on Sunday morning after a night of quarantine at sea, passengers lucky enough to have windows and balconies could see fire trucks and 15 ambulances waiting for the ship.

It was an unnerving sign for the nearly 3,700 people who had been confined for six days on the ship, which has become host to the highest concentration of coronavirus cases outside China.

That afternoon, the captain announced over the intercom that six more people — five of them crew members — had tested positive for the virus. Eight others would be taken off the ship to be treated for unrelated medical conditions, the captain said.

“Now we will start counting ambulances and know that’s the number being removed,” said Sarah Arana, 52, a medical social worker from Paso Robles, Calif.