China is growing "more aggressive" in dealing with rival territorial claims, the Philippines said Wednesday, after a fresh spat erupted between Tokyo and Beijing over a remote chain of islands.

"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.