WASHINGTON — The U.S. military has ordered the activation of a Navy hospital ship for possible deployment to the Philippines in December as the United States ramps up its relief mission after that country’s devastating typhoon, officials said on Wednesday.

The USNS Mercy is slow-moving and it could take about three weeks for the ship to reach the Philippines from San Diego if it first stops in Hawaii to pick up additional personnel and equipment, a U.S. military spokesman said.

With a capacity to treat hundreds of patients at any given time, the Mercy would bring enormous capability to efforts to help treat victims of Typhoon Haiyan during what is expected to be a long recovery for the Philippines.

Haiyan, one of the strongest storms ever recorded, tore through the central Philippines on Friday. It flattened the coastal city of Tacloban and destroyed all but two hospitals there.

The death toll is still unclear. But more than 670,000 people have been displaced by the storm, the United Nations said, and survivors have become increasingly desperate as essential supplies have dwindled.

Two days before the storm hit, the International Federation of Red Cross and Red Crescent Societies predicted a “dangerous” typhoon with 150-mph winds heading straight for Leyte and Samar — the two most devastated provinces.

Warnings were broadcast regularly on television and over social media. More than 750,000 people across the central Philippines were evacuated.

“As bad as the loss of life was, it could have in fact been much, much worse,” said Clare Nullis, spokeswoman for the U.N.’s World Meteorological Organization, praising the government’s work in issuing warnings.

But as the storm approached Tacloban and authorities crisscrossed the city, their warnings often fell on deaf ears.

“Some people didn’t believe us because it was so sunny,” said Jerry Yaokasin, vice mayor of Tacloban. “Some people were even laughing.”