NHK has learned that a hacking group called "BlackSuit" says that it stole data from major Japanese publisher Kadokawa through a cyberattack.
The group reportedly posted a statement on the dark web on Thursday claiming that it had gained access to Kadokawa's network.
Experts who checked the dark web say the group claims to have downloaded about 1.5 terabytes of data, including business plans and information related to platform users.
They add that the group is threatening to publish the data as early as July 1 if a ransom is not paid.
Kadokawa says multiple websites of the group are experiencing service outages after servers at its data center were hit by a major cyberattack, including ransomware, on June 8.
It says the incident has impacted the group's operations, including its video-sharing platform Niconico as well as its publishing business.
Kadokawa told NHK that it has asked experts to investigate possible information leaks.
It said no leakage of credit card data has taken place as it does not keep such information in its system.
The firm also said it expects to obtain more accurate information in July, and will make an announcement as soon as this becomes available.