TOKYO (Reuters) - Japanese Prime Minister Shinzo Abe ordered his defense minister on Tuesday to strengthen surveillance around islands at the heart of a territorial feud with China, Kyodo news agency reported.
Deputy Foreign Minister Akitaka Saiki summoned the Chinese ambassador earlier in the day to protest against an "incursion" by four Chinese maritime surveillance ships near the islands, officials said.
"I want you to respond firmly," Kyodo quoted Abe as telling Defence Minister Itsunori Onodera.
The ships entered the area around noon on Monday and left in the early hours of Tuesday, the officials said.
China's State Oceanic Administration confirmed four Chinese marine surveillance ships were patrolling waters near the islands.
But China routinely maintains such ships are in Chinese waters and a Chinese official accused Japan of intrusion.
"Japan has continued to ignore our warnings that their vessels and aircraft have infringed our sovereignty," the Communist Party chief of China's marine surveillance corps, Sun Shuxian, said in an interview posted on the Oceanic Administration's website.
"This behavior may result in the further escalation of the situation at sea and has prompted China to pay great attention and vigilance," Sun was quoted as saying.
Sino-Japanese ties chilled after the Japanese government bought the disputed islands, known as the Senkaku in Japan and Diaoyu in China, from a private Japanese owner last September.
Japan's Defence Ministry has scrambled F-15 fighter jets several times in recent weeks to intercept Chinese marine surveillance planes approaching the islands.
The hawkish Abe, whose conservative Liberal Democratic Party (LDP) returned to power in a landslide election victory last month, has vowed a tough stance in the territorial feud.
Reporting by Kiyoshi Takenaka, Hitoshi Ishida and Linda Sieg; Editing by Nick Macfie