The incident was "the first time four (Chinese) ships carrying what appear to be cannons entered Japanese territorial waters" around the Senkaku islands, Yoshimasa Hayashi told reporters.
The islets are also claimed by China, which calls them the Diaoyus, and the territorial dispute is a long-running sore point between the neighbours.
Relations suffered in 2012 when Tokyo "nationalised" some of the remote islands, and Japanese officials regularly protest at the presence of Chinese coast guard and other vessels in the surrounding waters.
On Friday morning, the four Chinese coast guard ships entered territorial waters controlled by Tokyo and left two hours later after repeated warnings from the Japanese coast guard, Hayashi said.
"The Chinese coast guard ships' intrusion into our territorial waters is a violation of international laws, so we have lodged a strong protest through a diplomatic route," he told reporters, adding the incident "can't be tolerated".
A Japanese coast guard spokesman confirmed it was the first time that all the ships in a group entering the waters were believed to have been equipped with such weapons.
Chinese ships have been repeatedly spotted near the disputed islets since 2012, but "in recent cases, only one of four Chinese vessels in a group had what appeared to be a cannon", spokesman Takanori Fukuda told AFP.
In August 2016, as many as seven Chinese vessels with what appeared to be cannons transited nearby, though not inside Japan's territorial waters, according to Fukuda.
Last month, Japan said it had spotted Chinese ships sailing near the disputed islands for a record 158 consecutive days.
Related Links
Naval Warfare in the 21st Century
| Subscribe Free To Our Daily Newsletters |
| Subscribe Free To Our Daily Newsletters |
