SINGAPORE - A United States Navy seaman reported missing since Monday (May 7) was recently sighted by his fellow sailors. However, they were unable to get to him, said a US Navy official on Thursday.

The missing seaman, Seth Austin Woods, 18, looked fine and was not distressed, said Vice-Admiral Phillip G. Sawyer, commander of the US 7th Fleet.

According to online discussion platform Reddit user jms428, who claims to be Seth's cousin, the sailor disappeared on Monday in Singapore near a 7-Eleven store during his submarine's routine crew rest here.

"His friend went to 7-Eleven and he went to a smoking area to vape. When his friend came back he was gone. It's been over 30 hours," wrote the user on Monday.

Seth, a machinist mate fireman, was assigned to a submarine on a routine scheduled deployment to the Indo-Pacific region. Submarines and other US Navy ships make routine visits to Singapore for crew rest, said Lieutenant-Commander Arlo Abrahamson, spokesman for the US 7th Fleet in Singapore.

He added: "The US Navy is proactively cooperating with local law enforcement authorities and making a thorough search for this missing sailor. We hope to return Fireman Woods to the ship soon."

Seth's mother, Ms Carmen Smith-Stow, last heard from her son on March 17, just before he left on the submarine, she told The Straits Times earlier.

"We are trying to come up with the funds to get (to Singapore) to help look for Seth," she said.

The Singapore Police Force has issued an appeal for information on the sailor.

Anyone with information is requested to call the police hotline on 1800-255-0000 or submit information online at www.police.gov.sg/iwitness

All information will be kept strictly confidential, said the police.