The talks of the Association of Southeast Asian Nations (ASEAN) and regional powers including China will be attended by US Secretary of State Hillary Clinton with the South China Sea likely to be high on the agenda.

Tensions have risen recently over territorial claims in the resource-rich waters amid a standoff between China and the Philippines at the Scarborough Shoal and between China and Vietnam over the Spratly and Paracel islands.

All parties must realise that "there is an enormous potential for developments that undermine the very confidence on which Asia prosperity is built", a US government official told reporters in Beijing.

"With the slowdown in Europe and some uncertainties on the recovery in the United States, it's clear that role of Asia is central," he added.

"The South China Sea issues are complicated by the fact that they stir intense nationalist sentiment in all of the countries involved."

Clinton, who paid an unannounced visit to Kabul Saturday, will attend a major development conference for Afghanistan in Japan Sunday, before travelling to Mongolia, Vietnam and Laos ahead of the talks in Cambodia.

China claims nearly all of the South China Sea, even waters close to the coasts of neighbouring countries.

Hitoshi Nakama, a local politician from Ishigaki, Okinawa, and another person went ashore on Kitakojima, one of a group of islands making up what Japan knows as Senkaku and China calls Diaoyu.

"Despite our warning, they dived into the sea from a fishing boat and reached the island," a Japanese coastguard official said, adding that the two stayed for 90 minutes before leaving.

The case is under investigation by both the coastguard and police, the official said.

The Japanese government controls the strategically-coveted but uninhabited islands and is resolute about the territory being Japanese, but prohibits people from going there without permission in a bid to cool tensions.

In Beijing, foreign ministry spokesman Liu Weimin hit out at the landing.

"The Diaoyu Islands and affiliated islands have been China's inherent territory since ancient times," he said.

"The illegal acts of the Japanese right-wing activists severely violate Chinese territorial sovereignty. The Chinese side has made solemn representations and protests to the Japanese side."

Taiwan reasserted its claim to the outcrops, and urged Tokyo "to exercise self-restraint and not to interfere with our sovereignty", according to a statement from the island's foreign ministry.

The archipelago, which lies in rich fishing grounds and may harbour valuable mineral reserves, was the scene of a nasty spat between Tokyo and Beijing in 2010 when Japan arrested a Chinese trawlerman who rammed two of its patrol vessels.

Tokyo's controversial nationalist governor has said he is in negotiations to buy the islands from their private Japanese owner on behalf of the metropolis, charging the government is not doing enough to protect Japanese territory.

Earlier this week Taiwanese vessels tangled with Japanese patrol ships in waters near the islands.