California Gov. Gavin Newsom on Thursday ordered the state's 40 million residents to stay at home, restricting non-essential movements to control the spread of the coronavirus that threatens to overwhelm the state's medical system.

"This is a moment we need to make tough decisions," Newsom said. "We need to recognize reality."

His move came after counties and communities covering about half the state's population already had issued similar orders. He said the restriction is "open-ended," and it could raise false hopes if he predicted how long the order might last.

People may still leave their homes for walks and exercise and for essential needs such as food and medical care. Restaurant meals can still be delivered to homes.

The Democratic governor also announced that he is mobilising 500 California National Guard troops to help with food distribution, but said they will be in place only for humanitarian reasons.

"I don´t believe the people of California need to be told through law enforcement that it's appropriate just to home isolate," he said.

Pennsylvania earlier Thursday ordered all "non life-saving businesses" to close across the state.

Newsom earlier in the day asked the president to deploy a U.S. Navy medical ship to help the state expand its medical capacity and warned that more than half of California's residents could contract the new coronavirus.

Newsom asked President Donald Trump to send the USNS Mercy Hospital Ship to the port of Los Angeles for use through Sept. 1, in a letter dated Wednesday. California has disproportionately aided people returning to the U.S. from foreign countries and needs the ship to help "decompress" its health care delivery system as infection rates climb, Newsom wrote. The ship is based in San Diego.

He said infection rates are doubling every four days in some parts of the state and issued the dire prediction that 56% of California's population could contract the virus over the next eight weeks.

His spokesman later confirmed that number does not take into account aggressive mitigation efforts underway across the state. Many large counties had already been issuing shelter-in-place orders aimed at keeping Californians confined to their homes and Newsom had directed the closure of bars, gyms and other gathering spaces statewide.

"This projection shows why it´s so critical that Californians take action to slow the spread of the disease - and those mitigation efforts aren´t taken into account in this projection," spokesman Nathan Click said in an emailed statement.

Newsom's letter to Trump said 25.5 million people could be infected. But the state's population is estimated to be just shy of 40 million, meaning 56% of the population would be closer to 22.4 million people. The governor's office did not respond to questions about his calculation or offer a prediction that considers efforts to stop the spread of the infection.