JAKARTA (NYTIMES) - When Indonesia recently - and quite publicly - renamed the northernmost waters of its exclusive economic zone in the South China Sea despite China's claims to the area, Beijing quickly dismissed the move as "meaningless." It is proving to be anything but.

Indonesia's increasingly aggressive posture in the region - including a military buildup in its nearby Natuna Islands and the planned deployment of naval warships - comes as other nations are being more accommodating to China's broad territorial claims in the South China Sea.

The two countries had three maritime skirmishes in 2016 involving warning shots, including one in which Indonesian warships seized a Chinese fishing boat and its crew.

Indonesia is challenging China, one of its biggest investors and trading partners, as it seeks to assert control over a waterway that has abundant resources, particularly oil and natural gas reserves and fish stocks.

The pushback from Indonesia takes direct aim at Beijing's claims within the "nine-dash line," which on Chinese maps delineates the vast area that China claims in the South China Sea. It also adds a new player to the volatile situation, in which the US Navy has been challenging China's claims with naval manoeuvres through waters claimed by Beijing.

Indonesia "is already a party to the disputes - and the sooner it acknowledges this reality the better," said Ian J. Storey, a senior fellow at the Institute of South-east Asian Studies in Singapore, where he researches South China Sea issues.

The dispute largely centres on the Natuna Sea, a resource-rich waterway north of Indonesia that also lies close to Vietnam's exclusive economic zone.

Before naming part of the contested waterway the North Natuna Sea "to make it sound more Indonesian," Storey said, Indonesia last year began beefing up its military presence in the Natunas. That included expanding its naval port on the main island to handle bigger ships and lengthening the runway at its air force base there to accommodate larger aircraft.

For decades, Indonesia's official policy has been that it is not a party to any territorial disputes with China in the South China Sea, unlike its regional neighbors Brunei, Malaysia, the Philippines and Vietnam.

Last year, however, Indonesia and China had the three maritime skirmishes within Indonesia's 200-nautical-mile exclusive economic zone off its Natuna Islands, which lie northwest of Borneo.

After the third skirmish, in June 2016, China's Ministry of Foreign Affairs issued a statement in which it claimed for the first time that its controversial nine-dash line included "traditional fishing grounds" within Indonesia's exclusive economic zone.

The administration of Indonesian President Joko Widodo, whose top administrative priorities since taking office in October 2014 include transforming his country into a maritime power, has ordered authorities to blow up hundreds of foreign fishing vessels seized while illegally fishing in Indonesian waters.

Joko, during a visit to Japan in 2015, said in a newspaper interview that China's nine-dash line had no basis in international law. He also chaired a Cabinet meeting on a warship off the Natunas just days after last year's third naval skirmish - a move analysts viewed as a show of resolve to Beijing.

On July 14, Indonesia's Ministry of Maritime Affairs and Fisheries held a conspicuously high-profile news conference to release its first national territorial map since 2005, including the unveiling of the newly named North Natuna Sea. The new map also included new maritime boundaries with Singapore and the Philippines, with which Indonesia had concluded agreements in 2015.

Arif Havas Oegroseno, a deputy minister at Indonesia's Coordinating Ministry of Maritime Affairs, told journalists that the new Indonesian map offered "clarity on natural resources exploration areas."

That same day, Indonesia's Armed Forces and Ministry of Energy and Mineral Resources signed a memorandum for warships to provide security for the highly profitable fishing grounds and offshore oil and gas production and exploration activities within the country's exclusive economic zone near the Natunas.

Gen. Gatot Nurmantyo, commander of the Indonesian armed forces, said at the time that offshore energy exploration and production activities "have often been disturbed by foreign-flagged vessels" - which some analysts took as a reference to China.

Although several countries take issue with China's territorial claims in the South China Sea, few do so publicly, and the Trump administration has recently sent mixed signals about how willing it is to challenge China on its claims. That has made the Indonesian pushback more intriguing.

Frega Ferdinand Wenas Inkiriwang, a lecturer at the Indonesian Defense University, said Indonesia's public naming of the North Natuna Sea "means that Indonesia indirectly becomes a claimant state in the area, perhaps due to territorial integrity issues.

"It's in the vicinity of the Natunas," he said, "and the Natunas contain natural resources which are inherited and will be beneficial for Indonesia's development."

Given that China is among Indonesia's biggest investors and trade partners, some analysts say Jakarta will go only so far in challenging China's territorial claims, at least publicly. But its more aggressive military posture and other moves regarding the Natunas are nonetheless sending signals to China.

"It doesn't make Indonesia a claimant state," said Aaron Connelly, a research fellow at the Lowy Institute for International Policy in Sydney, Australia, who follows the South China Sea disputes.

"They've never accepted the legitimacy of the nine-dash line, which is why they say there's no overlap" with its exclusive economic zone.