A Japanese zoo that seemingly caught more squirrels than it lost denied any mystery behind the mismatch Friday, saying it did not know how many animals it had because they "keep reproducing".

Tokyo's Inokashira Park Zoo originally said about 30 of the bushy-tailed rodents had made a break for it when a typhoon brought a tree down on their enclosure.

A week later keepers announced they had recaptured 38 animals and were continuing to receive up to five sightings a day, prompting speculation that wild squirrels were among their number.

But on Friday, zoo official Hiroshi Mashima said it was likely the number of runaway rodents had been higher than originally thought.

"We say more than 40 squirrels must have gotten away in the first place," said Mashima, a zoo educator specialising in animal biology. "There are no wild squirrels inhabiting this area. That is for sure."

Mashima said it was not possible to be precise because detailed records of the number of squirrels kept in the enclosure were not available.

The zoo does not know exactly how many squirrels it has "because they are difficult to catch and they keep reproducing", he said.

By Friday, 39 squirrels were back in captivity.