Indonesia sent four F-16 fighter jets to patrol its Natuna Islands, officials said Tuesday, as President Joko Widodo prepared to visit the area after asserting that Jakarta’s sovereignty in waters off the chain in the South China Sea was “non-negotiable.”

Tensions between Jakarta and Beijing have risen since last week after reports that dozens of Chinese fishing boats, backed by Chinese coast guard ships, had entered waters in Indonesia’s Exclusive Economic Zone (EEZ), prompting Indonesia to double its naval presence in the region.

“The four F-16s departed today,” Air Commodore Ronny Irianto Moningka, chief of the Roesmin Nurjadin Airbase, told reporters in Riau province, which includes Natuna regency.

“We have no intention of provoking any other parties. We only safeguard our territory,” he said, describing the scrambling of the fighter jets as “a routine operation.” Ronny did not say how long the patrol would last.

In Beijing, Chinese foreign ministry spokesman Geng Shuang told reporters Tuesday that “China and Indonesia have been in communication with each other through diplomatic channels.”

“China always views bilateral relationship with Indonesia from a strategic and long-term perspective,” he said. “We believe Indonesia will also have in mind the bigger picture of bilateral relations and regional stability, properly resolve differences with China, and foster favorable atmosphere.”

Beijing claims nearly all of the contested South China Sea, a vital sea lane through which about U.S. $5 trillion of shipping trade passes each year. Indonesia is not among the nations with overlapping claims in the sea, but Beijing says the waters off the Natuna islands are part of traditional fishing grounds for Chinese fishermen.

Brunei, Malaysia, the Philippines, Vietnam and Taiwan also have their own overlapping claims to portions of the sea.

‘We are not at war’

Meanwhile on Tuesday, Ryan Martinson, an analyst at the U.S. Naval War College’s China Maritime Studies Institute, wrote on his Twitter account that “currently at least four Chinese maritime law enforcement ships [were] operating in Indonesia’s EEZ.”

He identified the ships as “Zhongguohaijing, Haijing35111, Zhongguohaijian 2169 and a China coast guard vessel.”

On Monday, Martinson also posted on Twitter that the Chinese coast guard cutter Haijing 35111 had left the Fiery Cross Reef in the South China Sea and sailed in the direction of the Natuna islands.

The Chinese ship stopped over on one of the islands in the Spratly group, where China had refueling facilities, local reports said, citing data from Marine Traffic, a maritime intelligence provider with a website that tracks ships.

“This shows the value of China’s Spratly bases for grey zone coercion in the South China Sea,” Martinson tweeted.

Maritime Traffic also confirmed that Haijing 35111 and another ship, Zhongguohaijing, were in waters within Indonesia’s EEZ on Tuesday, about 200 km or 124 miles off the Riau Islands, local reports said. These also underscored that the vessels were within Beijing’s so-called Nine-Dash Line, a demarcation unilaterally placed by Beijing on Chinese maps to show its claims in the sea region.

The movement of the Chinese coast guard ships came as Jakarta dispatched four more warships to the Natunas on Monday, in addition to three warships and two aircraft deployed there last week.

Apart from the military ships, about 120 fishermen from the northern coast of Java island were being sent to the Natunas to strengthen the Indonesian presence in the region, according to Mahfud MD, Indonesia’s coordinating minister for political, legal and security affairs.

“We want to mobilize fishermen from the northern coast and maybe from other areas too to fish and conduct other activities there,” Mahfud told reporters Monday. “We are not at war, we are simply protecting our sovereignty.”

Indonesian Vice Adm. Yudo Margono, chief of the Joint Regional Defense Command, told reporters on Sunday that about 30 Chinese ships had been sighted in Indonesian territory.

“They were accompanied by two coast guard vessels and one Chinese fishing surveillance vessel,” he said.

Vice Admiral Achmad Taufiqoerrachman, chief of the Marine Security Agency (Bakamla), told reporters that Chinese fishing boats and coast guard ships were still in Natuna waters as of Tuesday morning.

“The Foreign Minister’s report said that there are still two coast guard [ships] around them,” he said.

Jokowi weighs in

Last week, the Indonesian government dismissed as “legally baseless” China’s historical claims to the waters off the Natunas and Jakarta summoned Chinese Ambassador Xiao Qian to lodge a protest over alleged violations.

During a cabinet meeting on Monday, Jokowi emphasized that the waters off the Natuna Islands could not be negotiated.

“On the Natuna issue, I think all statements [from Indonesian officials] so far have been good, that the sovereignty of our territory is non-negotiable,” he said.

Members of Jokowi’s “advance team” met on Tuesday with Natuna district chief Hami Rizal, along with other local officials, to prepare for the president’s scheduled visit, Antara said. Officials declined to give the exact date of Widodo’s visit, it said.

Reported by BenarNews, an RFA-affiliated online news service.