The announcement by the Japanese Coast Guard led the government to set up crisis management task forces within the office of Japanese Prime Minister Yoshihiko Noda and the National Police Agency, Kyodo News reported. China calls the islands Diaoyu Islands, which are also claimed by Taiwan.
Tensions between Japan and China over the uninhabited islands in the East China Sea, believed to rich in resources, have been rising since activists from both countries landed on one of the disputed islands last month to assert their country's sovereignty.
The situation worsened earlier this week after Japan announced it had purchased three of the five Senkaku Islands from its private owners for $26 million and brought them under state control, a move strongly protested by China which called the purchase "illegal and invalid."
Separately, China's official Xinhua news agency in a brief report said "two Chinese surveillance ship fleets" had arrived Friday at waters around the islands and started patrol and law enforcement there.
The report said it was the first time for the Chinese ships to patrol there after the Chinese government announced the base points and baselines of the territorial waters of the islands.
"These law enforcement and patrol activities are aimed to demonstrate China's jurisdiction over the Diaoyu Islands and its affiliated islets and ensure the country's maritime interests" Xinhua quoted a government statement as saying.
Xinhua also quoted a Kyodo report that Prime Minister Noda said Friday Japan "will take all possible measures to ensure security" following the arrival of the Chinese surveillance ship fleets.
In another report, Kyodo said small crowds assembled outside Japanese diplomatic missions in Beijing and Shanghai Thursday for the third straight day to protest the nationalization of the Senkaku Islands.
The Wall Street Journal, quoting analysts, said Beijing's response indicated China wants both to cater to widespread anti-Japanese sentiment in the country but also avoid damaging critical trade ties. The analysts said China has sent such nonmilitary vessels to the islands in the past.
"It's almost a form of ceremony, China's way of saying that it will be taking countermeasures if Japan moves on the issue," the Journal quoted Prof. Yoshihiko Yamada at Japan's Tokai University as saying.
China maintains the islands have been part of its territory since ancient times and that the islands were illegally occupied by Japan in 1895. Beijing has urged Japan to immediately repeal its decision to bring the islands under state control.
Japan says it has found nothing to support the Chinese claim and that the islands have been part of its territory since 1895.
In other developments, Shinichi Nishimiya, Japan's newly named ambassador to China, was hospitalized after collapsing, the Japanese Foreign Ministry said.
Nishimiya, who is to succeed the current ambassador Uichiro Niwa, was taken to a hospital because of his "health condition," but the ministry did not elaborate, CNN reported.
Kyodo quoted police as saying they found no signs of foul play and that they believed Nishimiya's collapse was apparently caused by illness.