Vietnam is guilty of a "sheer fabrication" in accusing a Chinese navy vessel of "firing" on Vietnamese boats that were illegally fishing off China's Xisha Islands last week, the Chinese navy said.

"The Chinese vessel fired into the sky two red warning signal shells, which burned out and extinguished in the air", an unnamed official with the Chinese navy said on Tuesday.

The Chinese vessel had not fired weapons, and the Vietnamese boats had not caught fire, the official said.

At 10:30 am on March 20, four Vietnamese fishing boats illegally entered and fished in waters off the Xisha Islands, over which China has had indisputable sovereignty since ancient times.

Before sending warning shells into the air, crew on the Chinese naval ship repeatedly demanded that the Vietnamese ships leave the area. Their warning efforts, which also included whistle-blowing and shouting, all failed.

The Vietnamese Foreign Ministry said on Monday that one of the fishing boats was "chased and fired at" by Chinese ships, and "its cabin was burned down".

The Chinese naval source said, "Ulterior motives are behind the so-called firing fabricated by Vietnamese authorities."

Li Guoqiang, deputy director of the Center for Chinese Borderland History and Geography at the Chinese Academy of Social Sciences, said that for "a considerable time" Vietnam had conceded China's sovereignty over the islands before staking its "illegitimate, rival claim".

China has consistently exercised jurisdiction over the Xisha Islands, and there has been no dispute over them, he said.

Vietnamese fishing boats have illegally entered China's territorial waters frequently in recent years. Incomplete statistics show that last year at least 303 of these boats intruded into waters off the Xisha Islands.

A Vietnamese ship was expelled from the area for illegal fishing on March 13 by China Marine Surveillance patrol ships.

The official added, "It is completely justified and legitimate for Chinese naval vessels to expel ships that illegally enter China's territorial waters, and to safeguard the country's territorial sovereignty, maritime rights and interests."

Sansha City, which administers the three island groups of Xisha, Zhongsha and Nansha, was set up in July.

Beijing strongly protested after Vietnam's National Assembly passed a "Vietnamese Law of the Sea" in June, a bill that claimed China's Xisha and Nansha Islands are "within Vietnam's sovereignty and jurisdiction".

Li, the CASS expert, said, "We are expecting Hanoi to take tangible measures to jointly ensure peace and stability of the South China Sea, yet Hanoi has not done this."

Zhang Haiwen, deputy director of the China Institute for Marine Affairs, said Hanoi has challenged Beijing on issues including fishing and oil exploitation. He said Beijing has shown sincerity and restraint over the issue, and Hanoi had "betrayed" a consensus reached by leaders of the two countries and the spirit of the Declaration on the Conduct of Parties in the South China Sea.

Meanwhile, another two CMS vessels, the Haijian 167 and Haijian 75, on Tuesday completed their patrols in waters surrounding the Xisha and Nansha Islands.

Their mission lasted 12 days and covered more than 4,000 nautical miles, the State Oceanic Administration said.

Xinhua contributed to this story.

zhangyunbi@chinadaily.com.cn