A teenager rescued from the rubble of Japan's monster earthquake in a rare feat of survival recounted today spending nine days trapped inside the wreckage unable to alert rescuers outside.

Jin Abe and his 80-year-old grandmother Sumi Abe were in the kitchen on the top floor of a two-storey wooden house when the 9.0-magnitude tremor struck on March 11, unleashing a massive tsunami that flattened entire cities.

The building collapsed with both inside but the 16-year-old was able to reach blankets, food and drink, helping them survive for more than a week, huddled together to keep warm.

"We found some water and snacks, so we ate them," he said on Monday from his hospital bed in the devastated coastal city of Ishinomaki in Miyagi prefecture.

"We heard people outside but we couldn't escape," he said in a frail voice.

The teen described being confined in a space "the size of a room", unable to stand up or walk around.

Finally on Sunday he managed to claw through the rubble and call out to rescue teams combing the earthquake and tsunami zone, and was airlifted to hospital along with his grandmother.

"I'm glad we survived," he said.

The boy's father Akira said he never gave up hope of seeing his son again.

"We all believed they were alive somewhere," he said. "He doesn't talk much, but I always thought he was a great man. This time he really proved it."

The tale of endurance offered a glimmer of hope for the relatives of other people still missing after the twin natural disaster.

"I feel stunned that the two people survived in really difficult conditions for such a long time. This miraculous news is very encouraging for people affected by the disasters," said government spokesman Yukio Edano.

But freezing temperatures in the disaster zone have dimmed hopes of finding more survivors.

Troops announced Saturday they had found a man thought to have spent eight days in a half-destroyed house, but it later turned out he was actually an evacuee who had simply returned to his home.