Vietnam on Thursday accused China of violating its sovereignty by landing two more test flights on an island in the busy South China Sea waterway, four days after China landed a plane on the same runway in the disputed territory.
The runway on Fiery Cross Reef is one of three China has been building for more than a year by dredging sand up onto reefs and atolls in the Spratly Islands.
Vietnam's claim to the area overlaps that of China, which claims almost the whole of the South China Sea.
Two large Chinese civil aircraft landed on Wednesday on an airfield that China "illegally" built on the reef, Vietnam said.
"This is a serious violation of Vietnam's sovereignty, threatening peace and regional stability, threatening security, safety and freedom of navigation and aviation in the East Sea," foreign ministry spokesman Le Hai Binh said, using the name by which Vietnam refers to the South China Sea.
"Vietnam resolutely requests China to immediately end similar acts and have no further violating acts," Binh said in an online statement.
Vietnam would defend its sovereign rights and jurisdiction in the area through peaceful measures, in line with international law and the United Nations charter, he added.
On Saturday, China landed a civilian plane on the same 3,000-meter-long runway in its first test in the Spratlys, which was also the first time it had used a runway in the area.
The United States, which has criticized China's construction of islands in the South China Sea, said after the first landing it was concerned that the flight had exacerbated tension.
Each year, more than $5 trillion of world trade is shipped through the South China Sea, where Brunei, Malaysia, the Philippines, Vietnam and Taiwan have rival territorial claims.