(Adds details about sailor)

By Idrees Ali and Phil Stewart

WASHINGTON, April 9 (Reuters) - A sailor from the aircraft carrier Theodore Roosevelt was admitted to the intensive care unit (ICU) in Guam after testing positive for the coronavirus last month, the U.S. Navy said on Thursday, as the number of coronavirus cases on the ship jumped to over 400.

The case comes after Thomas Modly resigned as acting Navy secretary on Tuesday, following a mounting backlash for his firing and ridiculing the commander of the Theodore Roosevelt, who pleaded for help stemming a coronavirus outbreak onboard.

In a statement, the Navy said the sailor was in a 14-day isolation period when admitted to the U.S. Naval Hospital Guam on Thursday.

A U.S. official, speaking on condition of anonymity, said the sailor was found unresponsive and received cardiopulmonary resuscitation (CPR). The sailor is in critical condition.

The number of positive cases on the nuclear-powered ship is now 416, up from the 286 positive cases the Navy reported on Wednesday.

Air Force General John Hyten, vice chairman of the Joint Chiefs of Staff, told reports at the Pentagon that the military needs to plan for the possibility of more coronavirus infections on the level of the Theodore Roosevelt.

"I think it is not a good idea to think that the (Theodore Roosevelt) is a one-of-a-kind issue. ... To think that it will never happen again, is not a good way to plan," Hyten said.

The general added that there had been a small number of cases aboard another aircraft carrier as well, the nuclear-powered Nimitz. Sailors assigned to four carriers have now tested positive for the coronavirus, the U.S. official told Reuters.

Modly's resignation occurred only after mounting pressure from Congress and a backlash from the crew, and followed Trump's own suggestion on Monday that he might get involved in the crisis — saying the Navy captain whom Modly fired was also a good man.

Captain Brett Crozier, who Modly relieved of command last week, urged more dramatic steps to safeguard his sailors aboard the Theodore Roosevelt in a four-page letter that leaked to the public last week. Crozier is later believed to have tested positive for the coronavirus.

Modly's controversial trip to Guam over the weekend where he ridiculed the commander of the aircraft carrier cost taxpayers at least $243,000.

During the trip, Modly questioned Crozier's character, saying at one point he was either "stupid" or "naive." After audio of his speech leaked, including expletives, Modly initially stood by his remarks. But he later apologized at U.S. Defense Secretary Mark Esper's request. (Reporting by Idrees Ali and Phil Stewart; Editing by Catherine Evans and Jonathan Oatis)

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