Indonesia has protested to Beijing over the presence of a Chinese coast guard vessel in its territorial waters near the disputed South China Sea, saying it marked a “violation of sovereignty.

The boat trespassed into Indonesia’s exclusive economic zone off the coast of the northern islands of Natuna, Indonesia’s foreign ministry said in a statement Monday. It did not say when the incident occurred.

“The foreign affairs ministry has summoned the Chinese ambassador in Jakarta and conveyed a strong protest regarding this incident,” it said. “A diplomatic note of protest has also been sent.”

The ambassador will report back to Beijing, but both sides have decided to maintain good bilateral relations, it said.

The Chinese Embassy in Jakarta could not immediately be reached for comment.

Local fishermen saw a Chinese coast guard vessel escorting fishing boats several times in recent days and then reported what they had seen to the Maritime Security Agency, media reports said.

Indonesia’s foreign ministry reiterated its stance that the country is a nonclaimant state in the South China Sea and that it has no overlapping jurisdiction with China.

However, Jakarta has clashed with Beijing before over fishing rights around the Natuna Islands and has also expanded its military presence in the area.

China claims most of the South China Sea, an important trade route believed to contain large quantities of oil and natural gas.

Beijing has been building artificial islands in the area, developments that have irked members of the Association of South East Asian Nations.

Brunei, Malaysia, Vietnam and the Philippines — all members of ASEAN — and also Taiwan have claims in the sea.