Japan's coastguard said it was "on high alert" Wednesday, a year to the day since Tokyo nationalised islands that China says it has owned for centuries.

Often-testy ties have soured dramatically over the last 12 months, with frequent confrontations between official ships from Asia's two largest powers.

On Tuesday, Tokyo said it had not ruled out stationing officials there, provoking an ominous warning from Beijing that Japan "must be prepared to bear the consequences of this provocation".

"We are on high alert as today marks the first anniversary of the nationalisation of the Senkaku islands," coastguard official Yuma Miyako told AFP, referring to the Tokyo-controlled islands claimed by China as the Diaoyus.

Since last September, official Chinese vessels have regularly traversed the waters -- China said Tuesday it had carried out 59 such "patrols" -- each time being warned off by Japanese ships, and the two nations' militaries have shadow-boxed in international waters and international skies.

Tokyo says it nationalised the islands as a way to take the sting out of a potentially explosive attempt to buy them by nationalists, who talked of developing them for tourism.

It was somewhat wrong-footed by the vehemence of Beijing's response, which saw violent protests erupt across China and diplomatic ties frozen, badly affecting a huge trade relationship on which both countries depend.

A change of government in Tokyo that made hawkish nationalist Shinzo Abe prime minister did little to soothe matters.

Xinhua Tuesday accused him of turning a blind eye to the nation's "beautifying of atrocious wartime crime", the latest in a long line of tongue-lashings Chinese state media has delivered.

Eight Chinese ships spent several hours in the islands' territorial waters on Tuesday and four remained in the contiguous zone on Wednesday, Japanese officials said.

Contiguous waters are maritime areas adjacent to territorial sea where a coastal state has certain limited rights.

"We are preventing Chinese official ships from entering our territorial waters, with our ships sailing very close to the Chinese ships," coastguard official Miyako said.

Under the United Nations Convention on the Law of the Sea, a nation can evict foreign military ships that enter its territorial waters. However, Miyako said, the rules regarding official ships, such as coastguards, are unclear.

"Therefore we are working in line with the Japanese government's policy of demanding they stay out of our territory," he said.

Chief Cabinet Secretary Yoshihide Suga on Wednesday repeated Tokyo's mantra that the islands "are an integral part of Japanese territory", but stressed Japan cherishes ties with China as "one of its most important bilateral relations" and was keeping the "door open" for dialogue.

Analysts say the row is unlikely to fade given China's rising power and an uncompromising approach from both sides.

"I suspect the dispute won't ever be resolved as long as they keep their current positions," said Yoshinobu Yamamoto, professor of international politics at the University of Niigata.

"It is wise and practical for the two countries to put it aside and shift their focus to other areas such as economic and regional issues," Yamamoto said.

On Tuesday, Suga said the government was "considering it as an option", when asked if Japan would station officials on the islands, but did not elaborate.

Chinese foreign ministry spokesman Hong Lei said Beijing was "gravely concerned" by the remarks.

"China's resolve to defend the sovereignty of the Diaoyu islands is firm, and we will not tolerate the Japanese side taking action to infringe China's sovereignty," he said.

"The Japanese side must be prepared to bear the consequences of this provocation."

Japan annexed what it says were unclaimed islands in 1895. It says China's assertion of ownership only came after the discovery of resources in the seabed at the close of the 1960s.

Beijing maintains that the islands have been its territory for hundreds of years and were illegally snatched by Tokyo at the start of an acquisitive romp across Asia that culminated in World War II.

Analysts have warned the presence of so many vessels and airplanes increases the likelihood that a slip by one side could lead to a military confrontation, with serious regional, and possibly global, ramifications.