Indonesia on Friday announced it would increase maritime patrols around islands in the South China Sea, a few days after Chinese coast guard vessels “trespassed” near the northern Natuna Islands.

Natuna has become something of a flashpoint between Indonesia and China since what the Indonesian Foreign Ministry described as “illegal, unreported, and unregulated fishing activities and violations of sovereignty by China’s Coast Guard in Natuna waters” in its formal protest to China on Wednesday.

This was not a case of a single Chinese ship venturing a bit too far into Indonesia’s exclusive economic zone. The Jakarta Post quoted Indonesia’s maritime security agency, the Bakamla, stating that “dozens of Chinese coast guard ships were found escorting traditional fishermen, whom the authorities suspect were illegally fishing in Indonesian waters that China has claimed as part of its traditional fishing grounds.”

“According to the agency, about 50 Chinese ships entered Indonesian waters for the first time on Dec. 19 and left the territory a day later. However, more ships returned on Dec. 24 under heavy guard by Chinese coast guard vessels,” the Jakarta Post added.

The Indonesians made it clear they would never recognize the notorious “Nine-Dash Line,” a line China drew on the map to give itself control over virtually the entire South China Sea. Although the Natuna Islands, like most other islands in the region, are often described as “disputed,” Indonesia insisted there is no real dispute over its ownership.

The Chinese Foreign Ministry unhelpfully responded to Indonesia’s complaint with a vague comment about sharing “normal” fishing activities with Indonesia in the Spratly Islands, which are located well to the northeast of the Natuna Islands.

Facing repeated questions about the matter, a testy Chinese Foreign Ministry spokesman, Geng Shuang, insisted that China recognizes no international court rulings or competing claims that would deny it full control over the region.

“Whether the Indonesian side accepts it or not, nothing will change the objective fact that China has rights and interests over the relevant waters,” said Geng.

“The so-called award of the South China Sea arbitration is illegal, null and void and we have long made it clear that China neither accepts nor recognizes it. The Chinese side firmly opposes any country, organization or individual using the invalid arbitration award to hurt China’s interests,” he added.

Indonesia fired back that it gives equally little credence to China’s claim that it has the right to fish off the Natunas because Chinese fishermen have been working those waters for a long time. The Indonesians dismissed China’s claims as “unilateral” and without a “legal basis” under relevant international laws.

The Indonesian navy announced on Wednesday that it would send two more ships to bolster its defenses around the Natuna Islands, matching them against “two or three Chinese coast guard vessels” currently orbiting Indonesia’s exclusive economic zone.

On Friday, the Indonesian military raised its combat alert status around the Natuna archipelago and said both naval and air forces have been deployed to the region.

“We are not only deploying ships, but also fighter jets. We are on full alert,” said Indonesian Maj. Gen. Sisriadi on Friday, adding that a permanent Maritime Information Center would be constructed to watch for further territorial violations.

The South China Morning Post quoted analysts who said Indonesia has become significantly more assertive in standing up to China’s bullying. They predicted China could back down in the short term, but not enough to completely defuse tensions over the archipelago and possibly not enough to keep Indonesia from tipping away from China at the upcoming Association of Southeast Asian Nations (ASEAN) meeting.