One in three Japan Coast Guard vessels is being used beyond its operational time limit, constraining the service's capacity at a time when its mission is growing in importance amid Chinese muscle-flexing around disputed islands in the East China Sea, officials said Saturday.

The Japan Coast Guard officials attributed the aging of the fleet to a slow replacement process following the last mass fleet expansion decades ago after Japan extended its territorial waters and fishing zone in 1977.

The coast guard has set a 25-year use period for relatively large patrol ships in its fleet, and a 20-year use limit for its smaller patrol boats. Of the total 366 patrol vessels 129, or 35 percent, had exceeded their operational time limits by the end of March.