A bear that was holed up inside an electronics factory in north-east Japan after attacking four people has eluded capture, apparently by opening a window by itself and fleeing under the cover of night, authorities said.

The bear's trail suggests it released the window latch and pushed it open, according to city officials. Before Wednesday's escape, the bear was seen opening a faucet to drink using its front paws.

"We believe it to be extremely intelligent," Fukushima Mayor Yuki Baba said, according to Kyodo news agency.

The bear entered a steel factory in Fukushima late on Tuesday and attacked four people before moving to the nearby electronics facility, where authorities had hoped to catch it.

But an attempt to immobilise it with a tranquiliser dart failed. Food traps also proved ineffective, as the bear was able to eat the fruit and honey baits without getting caught, Japanese media report.

Authorities said they tried turning off the lights to lure the bear into the traps on Wednesday night, but it escaped through a window which it is thought to have unlatched.

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