The whereabouts of a 27-year-old prison escapee remained unknown on Monday, with a weeklong search involving some 6,600 police officers on a small forested western Japan island failing to nab the convicted thief.

Hiroshima and Ehime prefectural police have been pursuing Tatsuma Hirao on Mukaishima Island in the Seto Inland Sea since his escape on April 8 from a prison in the city of Imabari.

(Police checking vehicles in Onomichi in Hiroshima prefecture)

Police searchers have come up empty despite Hirao leaving clues of his whereabouts including fingerprints on cars he has broken into and on trash left over from confectionary and milk products he has consumed.

The unique geographical character of the island, which has a population of around 20,000, has thwarted police, its forest cover and hilly topography has prevented officers from making an effective search, they say.

The search has also been complicated by the 22 square-kilometer island having over 1,000 vacant houses which police cannot enter without permission of their owners, many of whom they are unable to identify.

Without access to the vacant houses, police are forced to investigate only from the outside, a senior police official said.

With seven thefts having occurred in a 2.5 km-wide area of the island between last Monday and Friday, Hirao is believed to be in possession of some cash.

(Mukai Island (L) in Hiroshima Prefecture)

Some police officers have speculated Hirao may be surviving on food found in vacant houses as well as items left as offerings at graves. In addition, there are citrus-fruit trees and unattended orchards that he could access.

The narrowest point between Mukaishima and the main Japanese island of Honshu is only 200 meters, leading to speculation Hirao may attempt to swim across. But an official from a local ferry operator said it would be a bad idea, "The tide is fast and the water temperature is still cold. It would be reckless to swim." There are also four bridges that he could use.

Hirao escaped from Matsuyama Prison's Oi shipyard, a rare open-type prison facility in Japan. One of Japan's four prison facilities without perimeter walls, the shipyard has seen 19 other people escape since it was established in 1961, according to the Justice Ministry.

Island residents are fed up with commotion Hirao has caused, especially traffic jams at police checkpoints.

"It takes many times longer than usual to drive (around). I'm wondering how long this situation will continue," said Hiroyuki Mizuno, a 70-year-old resident of the island.