Indonesia's air force deployed four fighter jets to the South China Sea on Tuesday in a stand-off with Beijing after Jakarta protested over a Chinese violation of its exclusive economic zone.
The stand-off began in mid-December when a Chinese coast guard vessel, accompanying Chinese fishing boats, entered waters off the coast of Indonesia's northern Natuna islands, prompting Jakarta to summon Beijing's ambassador.
The issue has soured Indonesia's generally friendly relationship with China, its biggest trading partner and a major investor in Southeast Asia's largest country.
Fajar Adriyanto, the air force spokesman, said four F-16 jets had been conducting flights over the islands, though he also played down fears of any confrontation with Beijing.
"They're doing standard patrols to protect our sovereign area. It just so happened that they're patrolling Natuna," Adriyanto said. "We don't have the order to start a war with China."
The South China Sea is a global trade route with rich fishing grounds and energy reserves and China claims most of it based on what it says is its historic activity. But Southeast Asian countries, supported by the United States and much of the rest of the world, say such claims have no legal basis.
On Monday, Indonesia said it was mobilizing fishermen to the northern Natuna region and had deployed several naval ships.
Chinese foreign ministry spokesman Geng Shuang said on Tuesday Beijing had "opened diplomatic channels" with Indonesia since the latest incident, and said "both countries shoulder responsibility for maintaining regional peace and stability".
There has been no negotiation with the Chinese vessels as of Tuesday, Nursyawal Embun, director of sea operations of Indonesia's Maritime Security Agency, told Reuters.
According to data from Maritime Traffic, a ship tracking website, at least two Chinese ships — Zhongguohaijing and Haijing 35111 — were in waters on the edge of Indonesia's exclusive economic zone on Tuesday, approximately 200km (124 miles) off Indonesia's Riau Islands.
The ships were within China's unilaterally-declared "nine-dash line", which marks a vast expanse of the South China Sea that it claims, including large swathes of Vietnam's continental shelf where it has awarded oil concessions.
The China Coast Guard ship Haijing 35111 is one of a handful of Chinese ships that was involved in a months-long standoff with Vietnamese ships last year near the offshore oil block in the disputed waters, which fall within Hanoi's exclusive economic zone.
Luhut Pandjaitan, the coordinating minister in Indonesia's cabinet overseeing resources and investment, told reporters on Tuesday that Indonesia's sovereignty was not negotiable, despite China's economic importance for his country.
"I would not sell our sovereignty for investment, never," he said. "I'm not stupid."