China has made a “strong protest” about Wednesday's visit by a group of Philippine lawmakers to a disputed territory in the South China Sea.

Chinese foreign ministry spokesman Ma Zhaoxu told reporters in Beijing Wednesday that the Philippine side seriously infringed on China's territorial integrity.

Beijing claims that it has indisputable sovereignty over the South China Sea islands and their surrounding waters. But the Philippines and several other Asian countries have overlapping claims in the region.

The Philippine group led by lawmaker and social activist Walden Bello landed Wednesday on a South China Sea island known as Pagasa in the Philippines and Zhongye Dao in China.

The legislators, including two members of President Benigno Aquino's party, said the trip was intended to assert Philippine sovereignty in a section of the potentially oil- and gas-rich Spratly Islands.

A reporter who accompanied the group said they were greeted on the tiny island by Philippine soldiers and villagers, who joined them in raising a flag and singing the national anthem.

Tensions between the countries have been raised by several incidents in which Philippine fishermen and oil exploration vessels were harassed by Chinese vessels in waters claimed by Manila as part of its exclusive economic zone. In response, Philippine officials have begun referring to the area as the West Philippine Sea.

Vietnam, Brunei, Malaysia and Taiwan also have territorial claims in the important maritime zone. Vietnam's communist government has permitted rare street demonstrations for the past seven Sundays protesting China's behavior in the disputed waters.

Taiwan-based newspaper The China Post quoted the island's foreign ministry as reiterating Taiwan's sovereignty over the Spratly Islands, shortly after the Philippine group made a visit there Wednesday.

In Bali, Indonesia, Chinese and ASEAN officials announced Wednesday that they had agreed on guidelines for peaceful negotiations toward resolving the territorial disputes in the South China Sea. The non-binding guidelines are aimed at eventual implementation of a binding code of conduct in the waters and islands of the disputed maritime region.