A fisherman who says he spent more than a year drifting across the Pacific Ocean before finally making landfall in the Marshall Islands has gone back to hospital for more tests.

Jose Salvador Alvarenga made a brief public appearance today looking much weaker than he did earlier in the week.

He was greeted by about 50 officials and media at the Marshall Islands Resort hotel where he is staying. Sporting a haircut and a shave, he was helped into the room by two people while others stood by ready to help.

Speaking in a low voice, he thanked the government and his friends for their help, but declined to take questions. The Associated Press listened into the meeting by mobile phone and had it described by an official.

"He walked in but he needed a lot of help," the official said. "I would say he made an impression that is really now far more consistent with the journey he said he made. He was much weaker, extremely tired looking, much less animated and was speaking extremely slowly. He looked exhausted, frankly. Like someone who has run two or three marathons."

Mr Alvarenga's spritely appearance on Monday when he greeted hundreds of well-wishers in the capital Majuro after making landfall last week on a remote atoll had many questioning his story. He earlier told officials he left Mexico in late 2012 with another fisherman, who later died, for a day of shark fishing when a storm threw them off course and they began drifting.

Officials at the meeting said Mr Alvarenga needed to be taken back to the hospital for more medical checks and it would likely be three or four days before he was fit enough to travel back to El Salvador.

Also speaking at the meeting was the Marshall Islands foreign minister, Phillip Muller, and Mexican diplomat Chris Clay Mendoza.

Mr Muller said the Marshall Islands was happy to live up to its humanitarian obligations and do everything it could to assist Mr Alvarenga.

Clay Mendoza said he'd immediately departed from Manila for the Marshall Islands after initial reports indicated Mr Alvarenga was a Mexican national. He said he was now assisting El Salvador in its efforts to repatriate him.