Chinese authorities have continued construction of several military-grade airstrips in the disputed South China Sea region despite warnings from international leaders, the Associated Press reported Sunday. The ongoing construction on the group of territories referred to as the Spratly Islands could quadruple the number of airstrips available to the Chinese military in the region as the nation continues to assert its sovereignty.

The construction of the airstrips as well as other military bases could have “significant impact on the local balance of power,” Euan Graham, director of the international security program at the Lowy Institute in Sydney, told the AP. “In periods of tension, the intimidation value of air patrols from the islands would be considerable,” he said.

The Spratly Islands are a small group of mostly uninhabited islands in the South China Sea. Several countries in the region, including the Philippines, Malaysia, Brunei and China, have made claims to sections of the islands. China has by far made the largest complaints while covertly building artificial islands to bolster its land claim.

China’s actions in the South China Sea, including construction on the islands as well as military exercises nearby, have angered several international powers, including the United States. The U.S. has denounced Chinese activity in the region, continuing to treat the area as international waters and sailing U.S. Navy ships through regions that the Chinese have claimed are sovereign.

Allies of the U.S. in Asia Pacific have rallied together, including Japan. The Asian nation has long been a close diplomatic and economic ally of both China and the U.S., and Japanese defense authorities expressed solidarity with the U.S. policy in the South China Sea in November.

“The international community will not allow the unilateral changing of the status quo by force, and our country believes the same," said Japanese Defense Minister Gen Nakatani, as reported by the AP, adding, "The U.S. believes the same, too, and we agreed on this point."