The Japanese government on Monday formally protested the incursion by three Chinese patrol boats into the waters surrounding the disputed Senkaku/Diaoyu islands.

The vessels entered the waters of the small archipelago around 7 a.m. on Monday and remained there for about 15 minutes, according to a source with the Japanese Coast Guard cited by the Kyodo news agency.

The Japanese Foreign Minister filed a protest via telephone through Beijing's embassy in Tokyo, but Chinese authorities rejected it and insisted that they hold sovereignty over the islands.

The archipelago, which consists of five uninhabited islets and three barren rocks, are the focus of a serious territorial conflict between Japan, which is the territory's de facto administrator, and China.

Located in the East China Sea, the islands have rich shoals of fish and it is believed that they may also hold important petroleum reserves.

Tension escalated last September when the Japanese government acquired three of the islands from their Japanese owner sparking resounding protests in Beijing and Taiwan - which also claims them - and a wave of anti-Japan demonstrations in several Chinese cities.

The upsurge in the conflict has brought relations between Beijing and Tokyo to their lowest level in years affecting Japan's economic interests in China, its biggest trade partner.

U.S. Secretary of State Hillary Clinton said last week that while her government takes no position regarding the sovereignty of the islands, Washington opposes "any unilateral action" to undermine the status quo. EFE