China has rejected Vietnam’s claims of violating its sovereignty by landing a plane on the disputed Spratly Islands. The Chinese government also said that it “has indisputable sovereignty over the Nansha Islands and their adjacent waters,” referring to the islands by their Chinese name, BBC reported.

Hua Chunying , China’s foreign ministry spokeswoman, said in a statement that China will not accept Vietnam’s “unfounded accusations” and that the flight was conducted to test if the airfield the country built on the Fiery Cross Reef was fit for civil aviation, the report added. Chunying also reportedly said that China hoped Vietnam could work towards achieving “sustainable, healthy and stable” development of bilateral ties, Bloomberg reported.

China claims most of South China Sea, and apart from building facilities on islands it controls, while it has also been reclaiming land and building new islands from reefs to increase its presence in the area.

This has caused disputes with other countries in the region as well. In October 2015, the Philippines formally lodged a legal complaint against China’s actions in the South China Sea with the Permanent Court of Arbitration in the Hague, which made an initial ruling in its favor.

China rejected the court’s arbitration, but Vietnam has reportedly said it is considering similar legal action against it larger neighbor.

Meanwhile, the United States State Department encouraged “all claimants to actively reduce tensions from unilateral actions that undermine regional stability,” BBC reported.