China claims almost all the energy-rich waters but neighbors Brunei, Malaysia, the Philippines, Taiwan, and Vietnam also have claims.

Tension escalated when Beijing dispatched a research ship to conduct an energy survey in waters controlled by Vietnam in July.

"On the subject of foreign policy, including the East Sea issue, the General Secretary stressed the importance of maintaining a peaceful and stable environment, and resolutely fighting to protect Vietnam's independence, sovereignty and territorial integrity," the state-run Voice of Vietnam (VOV) said on its website.

The South China Sea is known as the East Sea in Vietnam.

Vietnam has good relations with China but should "never compromise" on its sovereignty and territorial integrity, VOV quoted Trong as saying.

The Chinese vessel, the Haiyang Dizhi 8, was continuing its survey in Vietnam's exclusive economic zone late on Tuesday, under escort from at least three Chinese ships, according to data from Marine Traffic, a website that tracks vessel movements.

Vietnam's foreign ministry has repeatedly accused the vessel and its escorts of violating its sovereignty and has demanded that China remove its ships from the area.

On Sunday, Vietnam pulled DreamWorks' animated film "Abominable" from cinemas over a scene featuring a map that shows China's unilaterally declared "nine-dash line" in the South China Sea.

The U-shaped line is used on Chinese maps to illustrate its claims, including large swathes of Vietnam's continental shelf, where it has awarded oil concessions.

In August, police broke up a brief protest outside the Chinese embassy in Hanoi over the survey vessel.

Trong has made more public appearances in recent weeks after suffering from an unspecified illness.

The 75-year-old has presided over a widespread crackdown on corruption in the Southeast Asian country that has seen several high-ranking ministers and politicians, including one Politburo member, sent to prison on charges ranging from embezzlement to economic mismanagement.

REUTERS