Vietnam and China have concluded two days of joint naval patrols in the Gulf of Tonkin, including a port call to China, as the two sides remain locked in a heated spat over disputed territory in the South China Sea, state media reported Tuesday.

Two boats from each country participated in the event Sunday and Monday, sailing more than 300 nautical miles in Gulf of Tonkin waters bordering Vietnam and China, Vietnam's People's Army Newspaper said. Earlier it reported the Vietnamese ships would pay a port call to China before returning home.

"Respecting the signed agreements is one of the factors that will promote the friendly and neighborly relations between two countries and ensure sustainable stability and security at sea," Col. Nguyen Van Kiem, deputy chief of staff of Vietnam's navy and commander of its naval ships in the patrol, was quoted as saying.

It marked the 11th joint patrol since 2005 between the neighboring communist countries, but it was unclear how long the exercise had been planned or whether it signaled any cooling of tempers. Relations between the neighbors have plummeted in recent weeks as both sides continue to trade diplomatic punches over run-ins involving territory in the South China Sea claimed by both sides.

On Tuesday, a newspaper published by China's ruling Communist Party ran a scathing editorial, warning Vietnam to back off.

"If Vietnam wishes to create a war in the South China Sea, China will resolutely keep them company," the Global Times said. "China has the absolute might to crush the naval fleets sent from Vietnam. China will show no mercy to its rival due to 'global impact' concerns."

China has been upset with Vietnam's welcoming of U.S. involvement to help resolve disputes in the South China Sea that Beijing believes should be settled one-on-one. The editorial said any attack on Vietnam would likely not create a direct conflict with the U.S. Navy. However, it said, "Even if some friction occurs, that is no reason for China to put up with Vietnam's unlimited vice in the South China Sea."

The U.S. has said the South China Sea, home to key shipping lanes, is in its national interest.

Hundreds of Vietnamese protested Sunday for the third straight week, yelling "Down with China!" as they marched through the streets of the capital, Hanoi. Many also carried signs demanding that China stop invading the believed resource-rich Spratly and Paracel islands, which are claimed all or in part by Vietnam, China and several other countries.

Vietnam and China have a long history of scrapes on the contested high seas, typically resulting in tit-for-tat diplomatic rhetoric.

The recent blowup has sparked a feverish response from Hanoi, which accuses Chinese boats of hindering its oil exploration activities within 200 nautical miles of its coast, claimed as its economic exclusive zone. China dismisses the argument, saying the clashes occurred near the disputed Spratly islands and that Vietnamese vessels endangered Chinese fishermen.