The US Navy relieved the commander of the aircraft carrier USS Theodore Roosevelt on Thursday, three days after a letter he wrote seeking the Pentagon's help in dealing with a coronavirus outbreak aboard his ship leaked to the public.

The removal of Captain Brett Crozier from the command of the 5,000-sailor vessel was announced by acting Navy Secretary Thomas Modly, who said the commander "exercised poor judgment by allowing his letter to find its way to the US media", where it attracted wide attention this week.

"It raised alarm bells unnecessarily," Modly said.

About 100 sailors on the ship, now docked in Guam, have tested positive for the novel coronavirus.

Democrats on the House Armed Services Committee criticized Modly's decision in a Thursday statement: "While Captain Crozier clearly went outside the chain of command, his dismissal at this critical moment－as the sailors aboard the USS Theodore Roosevelt are confronted with the COVID-19 pandemic－is a destabilizing move that will likely put our service members at greater risk and jeopardize our fleet's readiness."

In the letter dated on Monday, Crozier requested the removal and isolation of more than 4,000 sailors from his ship, writing: "We are not at war, and therefore cannot allow a single sailor to perish as a result of this pandemic unnecessarily. Decisive action is required now."

Modly told CNN after the letter leaked that the Navy was trying to move sailors off the carrier but that there was not enough room in Guam to quarantine the entire crew.

Crozier's letter put the Pentagon on the defensive about whether it was doing enough to keep the ship's crew safe, and it alarmed the families of those aboard the vessel, whose home port is San Diego, California.

Guam Governor Leon Guerrero on Thursday said the sailors were allowed to leave the military base and be quarantined in hotel rooms. He said the US Pacific Fleet ensured that sailors who tested negative for COVID-19 would be quarantined for 14 days where they would have no interaction with the local community and be strictly monitored. The Navy would deal with any sailor in quarantine who tests positive, Guerrero added.

As of Thursday, the United States had over 245,000 COVID-19 cases as the global total surpassed 1 million, according to Johns Hopkins University and the World Health Organization.

More than 6,000 deaths were reported across the US, with New York state reporting over 92,000 COVID-19 cases as of Thursday night, more than the total number in Germany, which topped 84,000.

In a tweet on Thursday, US State Department urged all overseas US citizens "not to delay travel home, as transportation options may soon be unavailable".

Long Xingchun, an adjunct senior fellow at Beijing Foreign Studies University's Academy of Regional and Global Governance, said the announcement may indicate that the US will take more stringent prevention and control measures.

"The announcement follows the closure of the border between the US and Canada. Now the new notice to overseas Americans suggests that the US may try to contain the outbreak by cutting off traffic and closing some commercial flights in the future as the cases in the country escalate exponentially," Long said.

On Thursday evening, the plane of the New England Patriots professional football team landed at Boston Logan International Airport with more than 1 million much-needed N95 masks ordered from China. The masks will go to front-line healthcare workers and first responders fighting the coronavirus pandemic in Massachusetts, New York and Rhode Island.

"This was a collaboration between the US and Chinese governments and private sector folks, and required a lot of support from many entities and agencies along the way," Massachusetts Governor Charlie Baker said.

"The story of this remarkable delivery is a window into the bilateral coordination to acquire lifesaving equipment needed to battle the coronavirus pandemic in the US," the Chinese consulate in New York said in a statement.

In an attempt to meet a nationwide outcry by state governors for more ventilators, the White House announced on Thursday that President Donald Trump is invoking the Korean War-era Defense Production Act to resolve supply-chain issues encountered in the manufacturing of ventilators.

"Today, I have issued an order under the Defense Production Act to more fully ensure that domestic manufacturers can produce ventilators needed to save American lives," Trump said in a statement.

The order will help domestic manufacturers "secure the supplies they need to build ventilators needed to defeat the virus", the statement said.

It comes as the nation's stockpile has only 9,500 ventilators, with only 3,200 more scheduled to arrive by April 13, the Federal Emergency Management Agency told a US House committee this week.

Ai Heping in New York and agencies contributed to this story.