All the sailors had previously tested positive for the virus and had gone through at least two weeks of isolation. Before they were allowed to go back to the ship, all had to test negative twice in a row, with the tests separated by at least a day or two.

On Saturday, a Navy official confirmed eight additional sailors had tested positive again. A day earlier the Navy had said in a statement that five had tested positive a second time. The Navy official was not authorised to speak publicly and requested anonymity.

That some crew were testing positive again has puzzled officials and raised questions about reintegrating troops into the military if a second infection were possible.

Also questioned was the accuracy of testing itself. In some cases infection can be at such a low level that it is not detected by the test, which could mean there were no relapses. Also, people could be cleared though their virus levels were too low for detection.

The outbreak aboard the Roosevelt was discovered in March. In port in Guam since then, the ship sent more than 4000 of its 4800 crew members ashore for quarantine or isolation.

Earlier this month hundreds of sailors began returning to the ship, in co-ordinated waves, to prepare to set sail again.

Australian Associated Press