A popular Japanese zoo has delayed its opening for the summer season after an employee told police he had disposed of his wife's body in the zoo's incinerator, local media reported.

Asahiyama Zoo in the northern city of Asahikawa was supposed to welcome visitors on Wednesday in time for Japan's Golden Week holiday period, after completing a three-week maintenance break.

But the city government says the zoo will now remain closed until Friday as investigations continue.

Last week, police searched the zoo grounds after the employee told them he had disposed of his wife's body in the zoo's incinerator, local media reported.

The incinerator was used to dispose of animal carcasses when they died.

Opened in 1967, Asahiyama Zoo is one of the country's most popular zoos, known for its unusual enclosures - featuring glass domes and overhead cages - that allow visitors to observe animals up close.

The zoo, which receives more than one million visitors every year, has been closed since 8 April for maintenance ahead of the summer season.

According to local reports, authorities have been searching for the employee's wife since a friend reported her disappearance to the police.

Apologising for the inconvenience, authorities also warned that the zoo may be closed without prior notice if the investigation calls for it.

Hirosuke Imazu, the mayor of Asahikawa city, told a press conference on Tuesday that authorities were now grappling with what he described as an "unprecedented crisis".

"No one could have predicted it," he said. "I am overcome with immense anxiety, and I am facing a crisis of unprecedented magnitude."

"We are making preparations to welcome you, so we hope that as many people as possible will come to the park."