China has rejected an effort by Vietnam to protect its claims in the South China Sea, saying Hanoi's assertions to an international panel studying the Philippines' dispute with China are illegal. Foreign Ministry spokesman Hong Lei dismissed the Vietnamese action on Thursday, describing its claims over the Spratly and Paracel islands - known in China as the Nansha and Xisha - as invalid. "China will never accept such a claim," Hong said. Vietnam sent a statement to The Hague's Permanent Court of Arbitration last week, making three main claims in opposition to China's stand on the islands. First, it recognised the court's jurisdiction over the case brought by the Philippines, directly contradicting China's position that the court had no such authority. Second, it asked the court to give "due regard" to its legal rights and interests in the Spratlys, the Paracels and in its exclusive economic zone and continental shelf when deciding on the merits of the Philippine case. Finally, it rejected the Chinese nine-dash line demarcation - the basis of China's claims to ownership over much of the South China Sea - saying that it was "without legal basis". On Chinese maps, the nine-dash line covers about 80 per cent of the 3.5 million sq km South China Sea. Xinhua reported Hong as saying: "China urges Vietnam to earnestly respect our territorial sovereignty and maritime rights and resolve relevant disputes regarding the Nansha with China on the basis of respecting historical facts and international law so as to jointly maintain peace and stability on the South China Sea." The court has given China until Monday to respond to the Philippines' case. But China said last week it would "neither accept nor participate in the arbitration" as a UN tribunal had no jurisdiction over territorial disputes. A regional source told the South China Morning Post that Vietnam's action "is as much to protect Vietnamese interests vis-à-vis the Philippines as it is directed against China". Lawyer Rodman Bundy, who has handled territorial sovereignty cases, said there was no legal basis for Vietnam's third-party statement. "In my view, it should be ignored by the tribunal given that Vietnam has no standing in the arbitration," he said, describing the move as "a cat-and-mouse game going on outside the strict procedure of the arbitration". Australian Professor Carlyle Thayer, who has studied the maritime disputes, said Vietnam's move opened the door for it to explain its interests and was thus "a cheap way of getting into the back door without joining the Philippines' case". China and Vietnam were embroiled in a stand-off earlier this year when China placed an oil rig in the disputed waters near the Paracels. China later removed the rig earlier than planned. Analysts said ties had returned to a more even keel since - until Vietnam's latest move. On Vietnam's move, a source said: "There is reportedly no consensus in the Vietnamese Politburo on this subject. This is probably as far as the Politburo is prepared to go."