"It looks like they are becoming more aggressive every day," said Philippine Foreign Secretary Albert del Rosario, whose own country is locked in a months-long dispute with China over a shoal in the South China Sea.

Beijing on Wednesday asserted its "indisputable sovereignty" over the uninhabited territory in the East China Sea after three Chinese patrol boats approached the islands, prompting Japan to summon the Chinese ambassador.

The dispute, which centres around islands in the East China Sea known as Senkaku in Japanese and Diaoyu in Chinese, is the latest territorial row involving China and its neighbours.

It comes as China and Southeast Asian countries struggle to make progress on a code of conduct to ease tension in the resource-rich South China Sea.

Tensions have flared recently in the area with both Vietnam and the Philippines accusing Beijing of aggression.

China claims essentially all of the South China Sea, home to vital shipping lanes and believed to be rich in oil and gas deposits. Taiwan and ASEAN members the Philippines, Vietnam, Brunei and Malaysia also have claims in the waters.

Foreign ministers from across the region are currently meeting in the Cambodian capital Phnom Penh for a week-long security forum which has been dominated by efforts to ease friction over the competing claims.

A joint statement by the 10-member Association of Southeast Asian Nations (ASEAN) has been held up as countries wrangle over whether to include a reference to recent incidents in the South China Sea.

Del Rosario told reporters in Phnom Penh it had been a "difficult" day, adding that he was still pushing for a mention of the tense situation in the Scarborough Shoal, a group of rocky outcrops also claimed by China.

Chinese Foreign Minister Yang Jiechi met Japanese counterpart Koichiro Gemba in Phnom Penh where he "reaffirmed China's principled position" on the islands known as Senkaku in Japanese and Diaoyu in Chinese.

"He stressed that Diaoyu Islands and their affiliated islets have always been China's territory since ancient times, over which China has indisputable sovereignty," said a statement from the Chinese delegation.

Japan summoned the Chinese ambassador in Tokyo as three Chinese patrol boats approached the chain of islands, which are privately held by Japanese owners.

The crew of the Chinese vessels, which have since left the islands' immediate vicinity, initially rebuffed Japanese orders to leave, Japanese officials said.

"We are conducting official duty in Chinese waters. Do not interfere. Leave China's territorial waters," the Chinese crews said, according to the Japanese coastguard.

The waters around the disputed islands, which are close to oil reserves, have been the scene of previous rows, including the arrest of a Chinese trawlerman in late 2010.

Wednesday's spat is the latest clash over disputed territory between China and its neighbours that threatens to overshadow attempts to smooth regional relations at an Asian security summit in Cambodia this week.

The 10 members of Southeast Asian regional body ASEAN have been trying to agree a long-stalled "code of conduct" for the South China Sea that would help settle overlapping claims in the resource-rich waterway.

The Philippines is leading a push for ASEAN to unite to persuade China to accept a code based on a UN law on maritime boundaries that would delineate the areas belonging to each country.

Manila also wants ASEAN to condemn a standoff last month between Philippine and Chinese ships over Scarborough Shoal, an outcrop in the South China Sea.

Yang urged Japan to adhere to agreements and understandings with China "in good faith" and said it should return to "the right path of managing differences through dialogue and consultation with the Chinese side."

Japan-China relations this year had seen "some acute problems", the Chinese foreign minister acknowledged.

China's assertiveness over disputed territories in the South China Sea, which is home to vital shipping lanes, is seen by analysts as pushing anxious neighbouring countries closer to the United States.

Beijing also recently angered Vietnam by inviting bids for exploration of oil blocks in contested waters, sparking protests in Hanoi.

US Secretary of State Hillary Clinton arrived in Cambodia on Wednesday to press for closer relations with ASEAN, part of Washington's strategy of "pivoting" towards Asia to challenge China's influence.

She is to take part in the ASEAN Regional Forum on Thursday, which brings together 26 nations and the European Union.