President Joko Widodo also headed Wednesday to the fishing-rich waters around the Natuna islands, which border the South China Sea, most of which is claimed by China despite competing claims from other Southeast Asian nations including Vietnam, the Philippines and Malaysia.

The Indonesian military said it had deployed eight warships and four jet fighters ahead of Widodo's visit in an apparent bid to assert its sovereignty over the region.

"I have said many times Natuna is our sovereign territory," Widodo told reporters.

"There is nothing to be debated...I hope this is clear."

A Chinese coast guard vessel was spotted in Indonesia's exclusive economic zone Wednesday, the government said.

"We have deployed eight warships," said Navy spokesman Fajar Tri Rohadi.

The air force said fighter jets had also been deployed.

"(But) our personnel have been told that we are not going to make provocations but rather protect our territory," said Air Commodore Ronny Irianto Moningka.

China's foreign ministry downplayed the incident and said there was "no dispute over territorial sovereignty" between Beijing and Jakarta -- though the two have "overlapping claims for maritime rights" in the South China Sea.

Indonesia does not lay claim in the South China Sea, but said it would not tolerate incursions by China -- a key trading partner -- into its nearby waters.

"We are willing to continue to properly handle differences with Indonesia," said spokesman Geng Shuang at a press briefing in Beijing on Wednesday.

"China and Indonesia have always maintained communication through diplomatic channels on this matter," Geng said.

Indonesia's move to send in warships follows the deployment on Friday of around 600 personnel from the navy, army and air force to Natuna as the military launched what it called a regular patrol to secure the area due to the presence of foreign vessels in Indonesian waters.

Jakarta said it would also send hundreds of fishermen to the area to keep an eye out for foreign vessels.

That followed Indonesia summoning the Chinese ambassador last week and lodging a "strong protest" over a Chinese coast guard vessel escorting Chinese fishing boats around the islands in mid-December.

Beijing responded that it has "historic rights" in the region and that fishing boats had been carrying out "legal and reasonable" activities.

Beijing lays claim to huge swathes of the South China Sea, where it is accused of building military installations and artificial islands -- and ramming fishing vessels.

China claims the majority of the resource-rich waterway through the so-called nine-dash line, a vague delineation based on maps from the 1940s as the then Republic of China snapped up islands from Japanese control.