A seaman with the United States Navy is said to have been missing in Singapore for almost two days.

Mr Seth Woods was last seen at a 7-Eleven store, a person claiming to be his cousin said on Internet forum Reddit Singapore on Monday.

"He and a friend got off the ship in Singapore," wrote the user jms428.

"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. They called his mother this morning. Just sharing for awareness and the hope he is found."

Lieutenant-Commander Arlo Abrahamson, the US Navy's spokesman for the US 7th Fleet in Singapore, told The Straits Times yesterday it was aware of the situation and is looking into the matter.

"The US Navy is cooperating with local authorities and we will continue to diligently search for this missing sailor," he said.

Mr Woods is 18, his sister, Ms Jodi Brown, told ST yesterday.

She posted photos of her brother in uniform and civilian clothes on Facebook, asking anyone who had seen him to contact her.

In a post yesterday, she wrote: "My brother is currently missing in Singapore... so if anyone hears from him please let my family know... We don't know much information but his shipmates are looking."

Ms Brown told ST that she had no reason to believe he had run away.

"He loved what he did," she said. Ms Brown did not know the location of the store he was reportedly last seen at.

Mr Woods' mother said she was told about his disappearance on Monday morning.

"The navy is all out looking for him," Ms Carmen Smith-Stow told ST. "He has not been in contact with us. The last time I heard from him was March 17, just before he left.

"I wish I knew more. It's killing all of us here, feeling helpless because we can't do anything."