After testing positive but asymptomatic, the five spent two weeks in quarantine in Guam where the nuclear-powered USS Theodore Roosevelt is docked, the official said, confirming a New York Times report.

After each registered negative twice in subsequent tests, they were permitted back aboard the ship.

But they were later retested and again found positive.

The Navy declined any official comment on the new cases.

The setback comes with the Roosevelt and its nearly 5,000 crew having spent the last six weeks at the US Navy's base on the Pacific island dealing with the outbreak.

More than one-fifth of the crew tested positive, including the captain, and were placed in quarantine as the vessel underwent a thorough cleaning.

The new positive cases underscored the challenges of testing and the persistence of the virus, which has caused nearly 8,300 infections and 27 deaths among US military personnel and civilian Pentagon employees and their families.

The Roosevelt sailed into Guam in late March and was forced to stay through April as the Pentagon faced its first real test from the pandemic.

The hobbling of one of the Navy's key assets in the Pacific raised questions about the threat of the global spread of coronavirus on US military readiness.

The Roosevelt outbreak led to turmoil at the top levels of the Navy: The ship's captain was removed after allowing his concerns over the outbreak to be made public, with the Navy secretary who fired him then resigning for mishandling the case.

A Pentagon official cautioned that the ship is now much less vulnerable to any new, large outbreaks because of continued testing, regular sterilization, use of masks and compartmentalizing of the crew.

Around 2,900 sailors have returned aboard the ship and all wear masks and maintain social distancing when they can.

"It's a very different environment than it was when this (first) outbreak occurred," according to the official.