A used Japanese fishing boat allegedly exported last August to North Korea without government approval is believed to have entered a port in the northeast of the country where two apparent spy ships fled after being chased out of Japanese waters in 1999, Tokyo police said Monday.

Public security officials of the Metropolitan Police Department said they suspect the fishing boat has now been modified for use by North Korea in spying activities.

The officials said the vessel is believed to be one of a number of used Japanese fishing vessels sold to the North by four men recently arrested over illegal exports to the country.