Japan said three Chinese surveillance ships entered its territorial waters off disputed East China Sea islands on Sunday, hours after one of Beijing's fisheries patrol boats sailed into the zone.
The three marine surveillance ships entered the 12-nautical-mile territorial zone off Uotsuri, one of the Senkaku islands, shortly before 1:00 pm (0400 GMT), the Japan Coast Guard said in a statement.
The incident came hours after Japan said a Chinese fisheries patrol boat briefly entered the territorial waters off another disputed island on Sunday morning.
Beijing claims the Japanese-controlled islands in the East China Sea, which it calls the Diaoyus.
The incidents were the latest in a series since Tokyo nationalised three islands in the chain in September, in what it said was just an administrative change of ownership.
The same Chinese fisheries patrol ship briefly entered the territorial zone on Saturday, as Japan's leader vowed he would not tolerate Beijing's incursions into the area.
Prime Minister Shinzo Abe, who was on his first visit to the United States since he took office in late December, said Japan "simply cannot tolerate any challenge now and in the future" to its control of the islands.
"No nation should make any miscalculation or underestimate the firmness of our resolve," Abe said Friday in Washington.
But speaking after White House talks with President Barack Obama, Abe also cautioned that "I have absolutely no intention to climb up the escalation ladder".