Tensions over the South China Sea continue to rise after the presence of Chinese military aircraft was reported to be patrolling the area.

According to Express, the fighters flew over the region near the Spratly Islands and even a refueling operation by one of the Flanker Su-30MKK fighters was recorded. The planes that were loaded with air-to-air missiles flew a 10-hour mission over the disputed area.

Aerial observers reported that the maneuvers of the People’s Liberation Army air force were presented as a response to recent naval exercises by the United States and its allies in the region.

The formation composed of six Su-30MKK Flankers seems to be assigned to the 6th Air Brigade, based in Suixi, Guangdong while the air base is located near the Chinese coast and the northern end of the South China Sea, according to The Drive.

Much of the area that the fighters flew over, where the disputed Spratly Islands are located, is recognized as international waters, and many of the islands in the vicinity have been claimed by neighboring nations such as the Philippines and Vietnam.

However, the Chinese Communist Party (CCP) persists in declaring a large area of the territory as part of its property, citing a territorial arrangement known as the “nine-point line,” which is based on a map from the Qing Dynasty indicating several islands in the area, including Taiwan, as part of China.

The claims by the CCP have also been reinforced after the construction of several landing platforms on the islands of the region. The pressure exerted by the CCP in the South Sea is due to the diversity of the waters for fishing as well as the possible presence of oil and gas reserves.

Recently the Chinese Communist Party gave the order to carry out drilling tests to identify underwater hydrocarbon reserves.

According to the Asian Times, the CCP would also be pushing foreign oil companies out of the South China Sea with the aim of becoming the only potential joint development partner for rival maritime claimants.

A U.S. State Department spokesman last year estimated that the CCP is effectively blocking the development of $2.5 billion worth of oil and gas resources in the South China Sea.

Secretary of State Mike Pompeo said in a statement issued last July, “Beijing’s claims of offshore resources in most of the South China Sea are completely illegal, as is its campaign of intimidation to control them.”

Another country that recently joined U.S. claims was Australia, declaring at the United Nations that “there is no legal basis” for the territorial and maritime claims made by the Chinese Communist Party in the South China Sea.

“Australia rejects China’s [the CCP’s] claim to ‘historic rights’ or ‘maritime rights and interests’ as established in the ‘long course of historical practice’ in the South China Sea,” it said.